Post a logfile from a live streaming or recording session where the issue is occurring.
Do be aware that if you are trying to stream a security system for monitoring purposes, there are much more appropriate tools than OBS which are purpose-built for exactly that, and are far less resource-intensive and have far lower hardware requirements. Immediately to mind is ZoneMinder, which I use for just that purpose with my own security system.
The blackscreen sounds like it is taking some time to acquire the videostream feed from the camera. Having both sources on the same scene and still having that delay supports this assumption.
Methods to correct this vary based on the capture source type in use; we'll be able to see that in the logfile though once you've posted one.
Global sources do still exist in OBS Studio, and default to on, but vary by source type and many do not initialize until they've been loaded at least once.
I have pasted the log file below, as requested.
This is to live stream two different bird box cameras to platforms such as YouTube, rather than for security purposes.
How can we make sure our media sources are global?
12:36:27.300: [ffmpeg] [udp @ 000001e2f2723f40] 'circular_buffer_size' option was set but it is not supported on this build (pthread support is required)
12:36:27.301: [ffmpeg] [udp @ 000001e2f2724540] 'circular_buffer_size' option was set but it is not supported on this build (pthread support is required)
12:36:27.449: [ffmpeg] [rtsp @ 000001e2e0968e80] setting jitter buffer size to 500
12:36:27.450: [ffmpeg] [udp @ 000001e2f2724040] 'circular_buffer_size' option was set but it is not supported on this build (pthread support is required)
12:36:27.451: [ffmpeg] [udp @ 000001e2f2723740] 'circular_buffer_size' option was set but it is not supported on this build (pthread support is required)
12:36:27.504: [ffmpeg] [rtsp @ 000001e2e0968e80] setting jitter buffer size to 500
12:36:27.852: [ffmpeg] [h264 @ 000001e2ed566600] Reinit context to 1920x1088, pix_fmt: yuv420p
12:36:28.759: [ffmpeg] [h264 @ 000001e2ed5673c0] Reinit context to 1920x1088, pix_fmt: yuv420p
12:36:45.113: User switched to scene 'Cam 02 Scene'
12:36:45.119: [ffmpeg] [tcp @ 000001e2f1b415c0] Starting connection attempt to 192.168.1.77 port 88
12:36:45.137: [ffmpeg] [tcp @ 000001e2f1b415c0] Successfully connected to 192.168.1.77 port 88
12:36:45.194: [ffmpeg] [rtsp @ 000001e2e09694c0] SDP:
12:36:45.194: v=0
12:36:45.194: o=- 1586950520860868 1 IN IP4 192.168.1.77
12:36:45.194: s=IP Camera Video
12:36:45.194: i=videoMain
12:36:45.194: t=0 0
12:36:45.194: a=tool:LIVE555 Streaming Media v2014.02.10
12:36:45.194: a=type:broadcast
12:36:45.194: a=control:*
12:36:45.194: a=range:npt=0-
12:36:45.194: a=x-qt-text-nam:IP Camera Video
12:36:45.194: a=x-qt-text-inf:videoMain
12:36:45.194: m=video 0 RTP/AVP 96
12:36:45.194: c=IN IP4 0.0.0.0
12:36:45.194: b=AS:96
12:36:45.194: a=rtpmap:96 H264/90000
12:36:45.194: a=fmtp:96 packetization-mode=1;profile-level-id=640028;sprop-parameter-sets=Z2QAKKw7UDwBE/LCAAADAAIAAAMAZQg=,aO484QBCQgCEhARMUhuTxXyfk/k/J8nm5MkkLCJCkJyeT6/J/X5PrycmpMA=
12:36:45.194: a=control:track1
12:36:45.194: m=audio 0 RTP/AVP 0
12:36:45.194: c=IN IP4 0.0.0.0
12:36:45.194: b=AS:64
12:36:45.194: a=control:track2
12:36:45.195: [ffmpeg] [udp @ 000001e2f2725240] 'circular_buffer_size' option was set but it is not supported on this build (pthread support is required)
12:36:45.195: [ffmpeg] [udp @ 000001e2f2724740] 'circular_buffer_size' option was set but it is not supported on this build (pthread support is required)
12:36:45.311: [ffmpeg] [rtsp @ 000001e2e09694c0] setting jitter buffer size to 500
12:36:45.311: [ffmpeg] [udp @ 000001e2f2723540] 'circular_buffer_size' option was set but it is not supported on this build (pthread support is required)
12:36:45.312: [ffmpeg] [udp @ 000001e2f2723840] 'circular_buffer_size' option was set but it is not supported on this build (pthread support is required)
12:36:45.335: [ffmpeg] [rtsp @ 000001e2e09694c0] setting jitter buffer size to 500
12:36:46.413: [ffmpeg] [h264 @ 000001e2ed568180] Reinit context to 1920x1088, pix_fmt: yuv420p
12:36:46.919: [ffmpeg] [h264 @ 000001e2ec0bd2c0] Reinit context to 1920x1088, pix_fmt: yuv420p
12:36:51.538: User switched to scene 'Cam 01 Scene'
12:36:51.552: [ffmpeg] [tcp @ 000001e2f1b41b80] Starting connection attempt to 192.168.1.74 port 88
12:36:51.604: [ffmpeg] [tcp @ 000001e2f1b41b80] Successfully connected to 192.168.1.74 port 88
12:36:51.706: [ffmpeg] [rtsp @ 000001e2e096a140] SDP:
12:36:51.706: v=0
12:36:51.706: o=- 1586950585300668 1 IN IP4 192.168.1.74
12:36:51.706: s=IP Camera Video
12:36:51.706: i=videoMain
12:36:51.706: t=0 0
12:36:51.706: a=tool:LIVE555 Streaming Media v2014.02.10
12:36:51.706: a=type:broadcast
12:36:51.706: a=control:*
12:36:51.706: a=range:npt=0-
12:36:51.706: a=x-qt-text-nam:IP Camera Video
12:36:51.706: a=x-qt-text-inf:videoMain
12:36:51.706: m=video 0 RTP/AVP 96
12:36:51.706: c=IN IP4 0.0.0.0
12:36:51.706: b=AS:96
12:36:51.706: a=rtpmap:96 H264/90000
12:36:51.706: a=fmtp:96 packetization-mode=1;profile-level-id=640028;sprop-parameter-sets=Z2QAKKw7UDwBE/LCAAADAAIAAAMAZQg=,aO484QBCQgCEhARMUhuTxXyfk/k/J8nm5MkkLCJCkJyeT6/J/X5PrycmpMA=
12:36:51.706: a=control:track1
12:36:51.706: m=audio 0 RTP/AVP 0
12:36:51.706: c=IN IP4 0.0.0.0
12:36:51.706: b=AS:64
12:36:51.706: a=control:track2
12:36:51.707: [ffmpeg] [udp @ 000001e2f2723a40] 'circular_buffer_size' option was set but it is not supported on this build (pthread support is required)
12:36:51.707: [ffmpeg] [udp @ 000001e2f2723b40] 'circular_buffer_size' option was set but it is not supported on this build (pthread support is required)
12:36:51.807: [ffmpeg] [rtsp @ 000001e2e096a140] setting jitter buffer size to 500
12:36:51.807: [ffmpeg] [udp @ 000001e2f2723940] 'circular_buffer_size' option was set but it is not supported on this build (pthread support is required)
12:36:51.808: [ffmpeg] [udp @ 000001e2f2723740] 'circular_buffer_size' option was set but it is not supported on this build (pthread support is required)
12:36:51.821: [ffmpeg] [rtsp @ 000001e2e096a140] setting jitter buffer size to 500
12:36:51.965: [ffmpeg] [h264 @ 000001e2ec0bd740] Reinit context to 1920x1088, pix_fmt: yuv420p
12:36:53.017: [ffmpeg] [h264 @ 000001e2ec0c0e00] Reinit context to 1920x1088, pix_fmt: yuv420p
12:36:56.466: User switched to scene 'Cam 02 Scene'
12:36:56.469: [ffmpeg] [tcp @ 000001e2f1b415c0] Starting connection attempt to 192.168.1.77 port 88
12:36:56.500: [ffmpeg] [tcp @ 000001e2f1b415c0] Successfully connected to 192.168.1.77 port 88
12:36:56.550: [ffmpeg] [rtsp @ 000001e2f3ec24c0] SDP:
12:36:56.550: v=0
12:36:56.550: o=- 1586950520860868 1 IN IP4 192.168.1.77
12:36:56.550: s=IP Camera Video
12:36:56.550: i=videoMain
12:36:56.550: t=0 0
12:36:56.550: a=tool:LIVE555 Streaming Media v2014.02.10
12:36:56.550: a=type:broadcast
12:36:56.550: a=control:*
12:36:56.550: a=range:npt=0-
12:36:56.550: a=x-qt-text-nam:IP Camera Video
12:36:56.550: a=x-qt-text-inf:videoMain
12:36:56.550: m=video 0 RTP/AVP 96
12:36:56.550: c=IN IP4 0.0.0.0
12:36:56.550: b=AS:96
12:36:56.550: a=rtpmap:96 H264/90000
12:36:56.550: a=fmtp:96 packetization-mode=1;profile-level-id=640028;sprop-parameter-sets=Z2QAKKw7UDwBE/LCAAADAAIAAAMAZQg=,aO484QBCQgCEhARMUhuTxXyfk/k/J8nm5MkkLCJCkJyeT6/J/X5PrycmpMA=
12:36:56.550: a=control:track1
12:36:56.550: m=audio 0 RTP/AVP 0
12:36:56.550: c=IN IP4 0.0.0.0
12:36:56.550: b=AS:64
12:36:56.550: a=control:track2
12:36:56.551: [ffmpeg] [udp @ 000001e2f2724640] 'circular_buffer_size' option was set but it is not supported on this build (pthread support is required)
12:36:56.551: [ffmpeg] [udp @ 000001e2f2725040] 'circular_buffer_size' option was set but it is not supported on this build (pthread support is required)
12:36:56.618: [ffmpeg] [rtsp @ 000001e2f3ec24c0] setting jitter buffer size to 500
12:36:56.618: [ffmpeg] [udp @ 000001e2f2723c40] 'circular_buffer_size' option was set but it is not supported on this build (pthread support is required)
12:36:56.619: [ffmpeg] [udp @ 000001e2f2723640] 'circular_buffer_size' option was set but it is not supported on this build (pthread support is required)
12:36:56.630: [ffmpeg] [rtsp @ 000001e2f3ec24c0] setting jitter buffer size to 500
12:36:57.782: [ffmpeg] [h264 @ 000001e2ec0bf740] Reinit context to 1920x1088, pix_fmt: yuv420p
12:36:58.227: [ffmpeg] [h264 @ 000001e2ec0c0500] Reinit context to 1920x1088, pix_fmt: yuv420p
12:37:02.029: User switched to scene 'Cam 01 Scene'
12:37:02.035: [ffmpeg] [tcp @ 000001e2f1b41b80] Starting connection attempt to 192.168.1.74 port 88
12:37:02.119: [ffmpeg] [tcp @ 000001e2f1b41b80] Successfully connected to 192.168.1.74 port 88
12:37:02.227: [ffmpeg] [rtsp @ 000001e2e09694c0] SDP:
12:37:02.227: v=0
12:37:02.227: o=- 1586950585300668 1 IN IP4 192.168.1.74
12:37:02.227: s=IP Camera Video
12:37:02.227: i=videoMain
12:37:02.227: t=0 0
12:37:02.227: a=tool:LIVE555 Streaming Media v2014.02.10
12:37:02.227: a=type:broadcast
12:37:02.227: a=control:*
12:37:02.227: a=range:npt=0-
12:37:02.227: a=x-qt-text-nam:IP Camera Video
12:37:02.227: a=x-qt-text-inf:videoMain
12:37:02.227: m=video 0 RTP/AVP 96
12:37:02.227: c=IN IP4 0.0.0.0
12:37:02.227: b=AS:96
12:37:02.227: a=rtpmap:96 H264/90000
12:37:02.227: a=fmtp:96 packetization-mode=1;profile-level-id=640028;sprop-parameter-sets=Z2QAKKw7UDwBE/LCAAADAAIAAAMAZQg=,aO484QBCQgCEhARMUhuTxXyfk/k/J8nm5MkkLCJCkJyeT6/J/X5PrycmpMA=
12:37:02.227: a=control:track1
12:37:02.227: m=audio 0 RTP/AVP 0
12:37:02.227: c=IN IP4 0.0.0.0
12:37:02.227: b=AS:64
12:37:02.227: a=control:track2