I create some icebreaker type games to get everyone interacting.
I try to have a bunch of fun stuff for people to do; good drinks, maybe some games, good music, and let things happen organically.
I just tell them they’re all my friends so they’ll get along.
I theme the party something wild that will get everyone talking.
I try to have a couple hangouts with people mixing up the two groups of friends before the party so that when the party happens it won’t be split in two.