Breaking up is hard to do.
If both members of the separating couple are close friends, then you will be put into an even more difficult situation. Don't feel guilty about maintaining relationships on both sides, but remember that you are going to have to strike the balance between being a supportive friend and taking sides. If one person is trying to press you to say something bad about the other, you might say something like, "I know that you're really angry, and I promise you that I will keep whatever you tell me in confidence, but I don't really feel comfortable saying something bad about Leslie."
Another case where you have to be careful about getting drawn into another couple's conflict is when you and your partner are friends with the splitting couple, and each of you is closer to a different member of the dividing pair. Here you have to be careful about trying to mediate as a team or having the dispute spill over into your relationship. If you or your partner get requests to influence or find out information from the other side, you might say something like, "I know you and Pat are going through a difficult time, and I absolutely want to be here to support you, but my partner and I have agreed not to intercede beyond supporting our friends." Remember that your duty as a friend is to be compassionate, supportive, loving, and objective when needed. It is not your job to be the mediator.
So here's hoping that all your friends have long and happy lives together, and thank you for listening to quick and dirty tips for a more polite life.