I'm not a huge fan of any "requirement" to get to know a metamour. I think as long as you are satisfied that there is full disclosire of the relationship with all partners involved, that should be enough.
In the case of LDRs you truly have to trust that everyone is being honest but I don't see a reason to even make contact.
Having metamours that all get along can be wonderful (that is my personal experience) but trying to force that state can sometimes take away from the core relationship (my personal observations).
This can be uncomfortable for some who need that connection with thier partner's partners. It then becomes a matter of how valuable the relationship is when weighed against the compromise of not getting your metamour needs met.
Playing the Game of Life with Monopoly rules.
Monogamy might just be in my genes
Poly Events All Over