darksilence, unlike you I AM new to poly, so take my advice for what you feel it's worth.
I would agree with redpepper in that it's probably safest to keep your distance unless they directly involve you. You care for both of them and certainly if they come to you for emotional support you can't just turn them away, but if necessary make it clear you don't want to play favorites. And make sure to let them know the relationship, theirs, yours, and everyone's involved, is important to you. If they realize how much you care, they'll realize you have the best interest of the group at heart.