Indeed, what they say is true. I remember being unpleasantly shocked at a birthday party for my former boyfriend Victor to see all four of his other exes in attendance. At one point, the five of us were even corralled into a group photo. Flash! Posterity recorded. What Victor hadn’t counted on was that we would talk—about him—and since we were all history, as they say, we had a lot of pent-up dish. By night’s end, the entire party was referring to our merry little band as “Victor’s Victims.” Probably not what Victor had in mind.
Back to your question! First of all, as a host, you need to think of your guests’ comfort. Just because you want to invite them all, doesn’t mean you should. Masked or unmasked bitterness, rage, or jealousy can quickly tank the best of parties. But if you’re committed to this potentially prickly guest list, be transparent. You don’t need to tell each guest who among their exes will be present, but if they ask, fess up. Then, it’s every man and woman for him or herself. Some will simply not show, but at least the others know what they’re getting into.
Likewise, if you’re having trouble choosing between two separated lovers, feel free to invite them both. Let them choose to attend or not. Or the exes can decide between themselves what’s appropriate. But again, if asked whether the other is invited or attending, tell them what you know.
As for seating arrangements, you’ve got to be kidding. You can’t just put a group of unhappy souls at a table and ask them to break bread together. Instead, make sure the party is big enough so that folks can hide in corners or behind a tree.