Well, the top strings would be a new tuning the OP would have to learn to navigate. There's something to be said for familiarity (as in, tuning and specs - scale length, string spacing at both ends, neck shape - of a traditional guitar neck), when one has to jump to and from roles onstage.
That being said, I've always thought an 8-string guitar/bass, tuned EAdgcfad, with a partial capo on the topmost six strings at the 2nd fret, could effectively be the ultimate both-in-one.
String spacing could be ever so slightly wider (than guitar standard) between the lowest capoed pair, and gradually more so from that point bassward.
Choice of scale would be tricky - I'd choose 30" overall, so that the "guitar" part is at 27". Well, either that or multiscale. The new Ibanez SRC6MS, if its neck were wide enough to serve as a platform (it isn't), would put the high E right at around 25.5", as Antonio de Torres intended (not coincidentally, double the length of a Stradivari violin).
The above is pointless reverie at this stage, and not directly applicable to the OP, who needs a low B.
(Heh, look: daydreaming poet and I didn't know it.)