Oh. Well, yes, your situation is different (but in a way that has nothing to do with the efficacy of cohabitation). I have a cousin with the same condition and she's married to somebody who also has the same condition. Whew! But it seems they're able to keep each other on a pretty even keel.
It did, in that she was afraid that we would get married, and I wouldn't be able to handle living with her episodes all the time while she tries to work it all out, and I would leave her. Neither of us ever want to get a divorce, we want to be married once and stay married. When we got married, that was it for us. So partly the cohabitation trial was also reassurement for her.
BPD isn't easy, but if you know what you're dealing with, it's okay. If you don't know what's going on though, it'll destroy you in short time. The fact that she knows she has a problem is half the battle, so I have hope..