No I'm not suggesting you fuck around in somebody else's relationship. Yes, that's a bad idea. I meant that you should be close enough to at least one of them to talk about a loving relationship with them. Are you close enough to one to let some affection lead talking about how you feel about them? Yes, if you don't know if they would be open to a third, you have to find out. A loving relationship with one makes the conversation possible and necessary, so you have to cultivate that. If they have never talked about something like that, it's going to be slow going and fraught with tension.
Even then you have to work at it.
I know in my own relationship with my fiancee, I was outwardly ok and supportive with her seeing her girlfriend of many years, especially since she was an old friend of mine, but it took a long time to really understand what that means and deal with my feelings of insecurity and jealousy.
Is it possible to set up a situation with one of them where you can cuddle up on a sofa or something and see if the affection progresses to a point where you can talk about how you feel about each other? (I know of a friend who approached another friend and said something like, 'I really love you both and would never do anything to come between you, but I'd give anything to sleep with you.' It's like this great compliment but leaves open the door to think about it. Some things are possible and some things are not, but you have to knock on the door.)
As I said before, good luck. I know how nervous you feel about it.