When SO (also a BS) and I first got together, we came up with a code word to use when discussing potentially triggery subjects. Okay, we didn't actually plan to use a code word, but it got used in one of the early, triggery conversations, so we kept using it. It helped, a lot. If either of us were in a place emotionally where we couldn't handle a heavy discussion, we said so. We also made sure we got back to that topic, though.
Communication is key. No topic is taboo, but the other person also gets a say in when the topic gets discussed.
We also made sure we had other things in common besides both being BS. And the BS thing - it's a part of both our individual histories, but it's not a part of our relationship. We trust each other, and while I've had triggers during this relationship with SO, it hasn't been because of anything he has done. A situation might have caused one, but it's my reaction to the past, not my present.