You need a DPDT switch, one with six posts. The switching mechanism doesn't matter, it just needs to have two sets of three. Connect the bass' output to one side, and the guitar's to the other side. That said, I imagine you might have some grounding issues with that setup, but A) I could be wrong, and B) I can't think of another way to do it.
Edit - On my diagram, "signal" would have been a better wording choice than output. So keep that in mind in case it's confusing.
So far only the above is confusing - LOL, the wiring for this unit is at this url -
I follow wiring diagrams well..not so well spoken and written directions. These units feel, play and sound great but I have been trying for months to resolve the issue of the neck selector switch. I really appreciate your help with this and if I get this resolved, all the other forums in my "favorites" are history.