How are you today?

[Wade]

Alright, here's what you do (I think I saw this on a sitcom once).

Invite them both but tell each of them that you decided not to invite the other because your friendship with... whichever, is more important to you. Then, recruit another friend to monitor one of them while you're occupying the other (radio earbuds will be necessary). Be sure to switch regularly.

Now this can go one of two ways. Either it's successful and you'll have to continue separate friendships with both while maintaining the lie that you're no longer in contact with the other for the rest of all your lives....

Or, the operation will fail, the two will meet at the party, realize what an elaborate set up you've concocted and appreciate all the work you went through to keep their friendships and you'll all have a good chuckle. Then of course, your friends will fight bitterly making everyone around them uncomfortable and everyone leaves early.

Good luck!