@bodo said:

should be able to play one mp4 (i know hap is better) ....???? are there any known incompatibility with izzy

Yes, but there are various ways that different software can spit out mp4 files. There are known issues with certain quirky things like embedded timecode tracks and the h265 codec that can cause issues in Isadora, which is why our first suggestion is always to transcode any media files that are misbehaving in Isadora.

@jrhooker

I agree with @woland re HDMI emulators. It can help to stop this kind of glitch where if one connection falters it affects them all. But not always.

These ATEN ones are cheap, cheerful, and effective.

It's curious that the Viewsonic's connection drops only when both projectors attached. Do you have it set to the HDMI input, or is on auto-find connection? 

Who makes your USBC to HDMI dongles? IME cheap ones can be problematic and not even the HDMI emulator will solve an issue with them.
I've never encountered the OREI brand before. I see they are based in Illinois. You could reach out to them and see if they have a suggestion.

HTH Mark (not that Mark)