It does appear that I can keep both 1.1 and .9 Beta installed at the same time. As long as I choose "EOS Webcam Utility Beta" in Settings > Device it appears to work fine in Teams.
If you pick "EOS Webcam Utility", it'll crash in Settings and you'll have a hard time choosing a different camera as it will keep crashing after the application re-starts and you re-enter settings - but only after a few seconds. If you're
fast you can try to change the camera back to one that works (the Beta entry in the last, or another camera if you have one) in that small window of time before settings crashes, but it may take several attempts.
I assume using the camera switch button on while in a video call/meeting would be dangerous as it cycles to the next camera "source" without letting you choose. I bet it'll crash as it picks the non-beta EOS camera source. Just avoid the button I guess.
Not a big deal if you only have the EOS camera as your only webcam, if you have multiple camera this could be an issue as now you can't change it during a meeting. I doubt anyone does this normally? I used to before my I got an AC adapter for my Canon, I'd
switch to my built-in camera during long meetings when the DLSR ran out of battery.
Edit: After many meetings today (hours) I can confirm that the Beta driver in Teams works when installed alongside of 1.1. I just have to be sure to never switch to the release EOS camera source
while in Teams.