can you leave a mewtwo raid and help a second group?

I think that people have told me that with the damage staying with you when you faint out and come back likely means that the damage stays even if you exit all the alway out BUT I have never proved this. I would test the assist and then leave strategy with a L3 duo or L4 foursome raid to determine conclusively if the damage done by those who leave disappears or not.

Note that we tried this strategy week 3? of EX but someone from the leaving team didn’t actually leave. It was a kind gesture to help the minor team get a better shot and I commend you for your thoughtfulness.

To answer your question, Yes.
You can help the first group and back out all the way before they kill the Mewtwo to join your second/main group to complete your raid. I have had 6 ex raids so far, and each raid when certain teams don't have enough members (usually yellow.. sorry no offense), we join their private group to help them out and back out before Mewtwo goes down.
As for the damage bonus, i have not confirmed how many balls the other team receives since i can't see the completed raid screen on my part after i leave their raid to join mine.. but i will def check next time. Hopefully this helps.