Assuming you want to stream at 1080p, just make sure you set your Base (Canvas) and Output (Scaled) resolutions to 1920x1080 in Settings > Video.
When you add the 720p camera to a scene you may have to right click the source and choose Transform > Fit to Screen to fill the whole screen. OBS will take care of the upscaling on that camera for you. You’ll have to decide if this results in an acceptable looking picture.
If you want to stream at 720p set the above settings accordingly and OBS will scale down the 1080p camera for you.
