I run both a bass stack and a guitar combo together as my rig with my current band... I don't run the guitar amp through a bass cab, I just use its own speaker, because I'm only using it for highs, high-mids, and overdrive (I have the lows turned all the way down on it).
I've played guitar through guitar amps running bass cabinets before, and it sounds a little bit quieter than with proper guitar cabs (I guess they take more power to run), but defintely deeper.
I've also recently plugged my bass into my 1961 Ampeg Reverberocket guitar combo, which is like 17 watts on a good day... I had a lot of fun with that! I need to record a soundclip... I hope to use it for recording at least one track with my band
One thing I will never do is plug a bass amp into a guitar cabinet, even if the power handling numbers match up. I just would not trust that it would turn out OK for the cab...