I searched high and low for what I thought would be an obvious solution, but found nothing. Here is what worked for me. Add you videos as a source. In my example, I had 2 videos (PiP). Go to the "settings" or "preferences" to change the hot keys. In the recent OBS version, there is a set of hot keys for each video source. Change the "play" hot key to "p" (what ever you want, but don't use "space bar"). Do this for each source hot key. OBS will warn you that the keys are already used. But this is what you want. Set up your scene, move the slider to where ever you want to start the playback in each video. When you're ready, hit "record' then press your hot key and all videos will start playing.
