My personal feeling tends to be that when the people on the legs of the vee know each other and can be friendly the whole structure is more stable.
I have experienced this. My boyfriend of five years had a relationship with someone whom I just could not like, could barely tolerate. We had a lot in common but commonalities don't make a relationship all on their own, their has to be attraction on some level as well.
On the flip side he also had a relationship with someone I liked very much. We didn't have as much in common but there was the attraction there that couldn't be ignored.
He's currently interested in a co-worker and they're experimenting to see where things will go, or if they will.
Breathes (live in boyfriend) & my Possibility know each other and like each other &, in fact, are out right now playing D & D together as I write this.
Liking your significant other's oso really makes things run more smoothly. It's possible to have other relationships without having your partners get along but it sure does complicate things immeasurably!