How do you deal with a situation from both sides?

As this is a quite vague question, I’ll give some background regarding how it immediately pertains to me.

Yguy is my immediate coworker; he sits next to me. That is to say, we spend 40 hours per week working side by side. We are involved in a number of projects together, but our conversation topics extend well beyond work. We take hour+ breaks from actual work to discuss our personal lives with each other, have lunches together, and go out for drinks together after work. We have shared a great deal with each other, and he has been instrumental in helping me through emotionally trying situations in my life. We are not romantically involved. We are both in committed relationships, and he and his girlfriend frequently socialize with me and my boyfriend. Our two relationship pairs get along quite well. Basically, I am very close friends with Yguy.

Xgirl is my “coworker,” although not immediately. According to meeting schedules, we only run into each other approximately once per month at work. However, she is in my immediate social group (that is, the group of social twenty-somethings that spans departments and has similar senses of humor). She is a girl quite like me, I enjoy her company a lot, and I can see myself being good friends with her. We socialize regularly (though certainly independent of Yguy!), and we have a lot in common. We have in-depth conversations, and we frequently plan outings together, but I usually feel awkward about going through with the outings as a result of my friendship with Yguy…

Xgirl and Yguy had a thing awhile back, and it didn’t work out so well, to say the least. In fact, it resulted in so much tension that I notice Yguy hiding behind me and ducking out of the room during those monthly meetings in which Xgirl is present (she’s also doing the same!). From everything I can tell, it simply seems to have been a failed relationship gone miserably, terribly awkward.

They both hate each other, and I don’t want to take sides. Yguy has mentioned it to me on several occasions, and he’s given me brief details, but it has never been the pressing matter of discussion. He has always said, “It’s quite a story, so one day, we’re going to have to devote a few hours to me telling you the story of what happened with Xgirl.” I always left it at that. However, if I were to ask at any point (or even call him up at 2am on a Wednesday night and ask for the story), I am positive he would divulge willingly and completely.

Most of my immediate social group, however, is related to Xgirl. Tonight, my boyfriend and I started out attending a social event at Yguy’s house (btw, I am also friends with Yguy’s girlfriend), and we later ended up at a bar with my general social group, including Xgirl. Xgirl was slightly drunk and in the bathroom when I went to pee. After having peed and at the sink, she started talking to me about Yguy. The conversation ended up being so long that everyone in our social group was concerned/confused that the two of us were in the bathroom for more than an hour. Now that she has divulged the entire story to me from her side, I feel for her, but I also feel loyal to my good friend, Yguy.

When Yguy tells me the story of Xgirl (which I expect to happen within the next month), how do I respond? Do I act as if I haven’t heard her side?
How do I stay friends with both? Can I, or will that diminish my friendship with one or the other?
Is it despicable of me to listen to both of their sides of the story and pretend I don’t know the other? When they ask me if the other has mentioned anything, am I supposed to say “no”?
How do I handle this situation?