I don't have any other boundaries outside safer sex stuff than this one, my friends. I feel exactly like you do. I would be worried about the friendship. I admit it, it would bother me a lot if my partner dated my close friend. This is a boundary I'm willing to negotiate though, and in the end I probably wouldn't stand in the way of their relationship if that's something they both really wanted. But man, it would be so hard for me. I haven't been in your situation, but just wanted to tell you that you're not alone in your thoughts/feelings about this.
My partners: rory
My metamour: Alec (rory's partner)
Others: I'm dating Maxine, rory is dating Lily and Hank is dating Janet - all in early stages