Most Helpful Girl
You don't pick sides and you don't talk about the other to either of them. You invite them both and if one of them has an issue about it, that one can figure out what he is going to do.
Ultimately the issue is theirs not yours and they will each figure out how they would like to handle it. But it isn't worth burning the bridge on either side.
My great-grandmother had a rule about her sons' divorces "you had a problem and divorced her, I didn't. " As such I have several great aunts who have been divorced from my uncles for at least 30 years, longer than they were married, that are still very much a part of the family and are invited to EVERYTHING. Their ex husbands went on to marry and have kids with other women but all of them get along and attend family events together with no problems.
Once the initial emotions of the divorce have subsided: they probably won't mind being in the same room.