I have two friends who are in a relationship with each other and live together. Sometimes they invite me to their house and they say it's just to have fun. But every time I visit there's either a fight or very tense atmosphere between the two at some point during the night. When I'm alone with one, she (they're same sex) will tell me secrets and vent about the other and so on. Basically, I'm the verbal punching bag and this seriously happens every time I'm there. An example would be when I was alone with Tiffany, she would ramble on about how Jessie treats her like shit and how she hates Jessie's crush on the lead singer of Paramore. She feel so passionate about this that she actually gets pissed when Jessie decides to listen to the band's music. That same night, when I was alone with the other one, she felt compelled to tell me about her crushes from school and how she hates her girlfriend's body. Every single time it's something like this. If I decline their invitation, they post statuses up on Facebook like, "I'm so GLAD (sarcasm) to see that some friends want to keep in touch," and it pisses me off. Am I being a bad friend and do I have reason to be mad? Or am I the one being immature?
You friends are being pretty immature. You deserve better friends than that, but you can barely avoid them. One option is to be a mediator between them, but you might not want to. I wouldn't want to stand in the middle of them fighting either. A bad option, but an option none the less, you could just ask what they like about each other, they may get to thinking and it might work. I would be careful because they might break up soon, and try to blame it on you.. The next time they start to vent, engage them in conversation and see if they are willing to come to a solution in the middle. Try and get them to see the others point of view on that subject. I wouldn't alter their relationship too much because they live in the same house. Another idea is that anytime one of them starts to use you as a punching bag find a way to throw in a problem of your own, maybe she will find you talking about your own problem annoying, and she will maybe realize that venting everything to you is unfair. That's all I have for now, but hang in there.