Recently I discovered Charlie Hunter, he's a great musician who plays a 7 string bass guitar. Basically the three bottom string are where he plays the basslines and the top 4 are where he plays the guitar parts. I have a 12 string and I've been wondering if it's at all possible to somehow mimic this sound of Charlie Hunter? Here's a link of his playing:

P.S. If you like his playing, check out his song called "High And Dry". That's my favorite so far in terms of melody and rhythm .
I think it sounds like some sort of keyboard together with a bass! But if the separate bass part together with a melody and harmonies ontop is what you are looking for you should look for fingerstyle guitarists who plays in that way.

For example http://www.youtube.com/watch?v=0urEdeRQi1M or http://www.youtube.com/watch?v=z6UIXEDZGmg

But you won't need an 12 string to play that, and it doesn't really sound like Charlie. But I guess you perhaps could find a way with that style to make it sound like him, with like maybe using alternate tunings and effecs But I don't know though :P Good luck!