I'm curious about this too. I don't have any good insight for you. I honestly think my ex has narcissitic personality disorder. Not saying that just because he's my ex- we actually still get along ok sometimes. But really he fits it totally. And we had a LOT of problems. (i had no diagnosis when we were married but totally see the BPD now.)
I think- and only based on speculation, that it can have it's good and bad points. Good in that you'd really understand each other, bad because like you said... its can be an explosion waiting to happen.
I'd say if you both went to therapy seperatly, and then some together, and were willing to work at it- it might be able to work. Good luck whatever you decide. You dont want to ruin a friendship.