This is an interesting situation. Normally you'll find yourself asking a guitar player to play bass because you just can't find a bass player!!

From my experience, drums and bass are the hardest slots to fill, so things are good there.

If the second bass player doesn't agree with taking on guitar duties, do everything you can to leave things on a positive note...because you may find yourself calling him to be your bass player if things with the current one don't work out. You just never know what the future holds.

Be up front and don't lie to anybody. They'll understand that a band doesn't need 2 bass players. You may have to explain why you perfer one bass player over the other - that could be tricky. I know of bands that will switch out personnel based on availability. You might need to be more established for that to work.

Maybe somebody with more experience can chime in...