That won't be easy to do- You would be trying to cover too many frequencies with one mic, and probably won't get either to sound decent.
I suggest you get a mic for your guitar, Sennhieser E609, or Shure SM57, etc. for a hundred bucks or less.
I also recommend you get an inexpensive direct box for the bass,($35 or so) it will sound better than a mic anyway.