Taz is better at server tech than I am. I doubt you can install BB on one sub and then use SSI to call BB from another sub.
While I'm not a server tech expert, I do know something about SEO...So, why are you using subs instead of directories? Reports are, Google recently started treating sub domains the same as sub folders.
Since there's no obvious advantage to using sub domains, I suggest you just use sub directories...Plus, sub directories can all use the same install of BB using SSI tags.