For band practice I'm trying to setup 2 microphones that can amplify our vocals. We have a large bass amp we can use and from what I have read you can use that for vocals. Is there a way to drive two microphones into a bass amp?
I guess there's only one jack input on the bass amp. Some will have two - one for active and one for passive. With just the one, ideally you need a little mixer, with two inputs for mics. Relatively cheap - I just sold one for £15. But while you're at it, it may be worth getting a mixer that'll cope with 3 or 4 mics - always useful for later in he progress of your playing.
If you use two standard microphones, you will also need two standard XLR microphone cables, two XLR female-to-1/4 inch mono male jack cable adapters, and a single splitter that has two female mono jack receptacles with one 1/4 inch mono male jack output.
In the end, you will plug the single mono jack into the solitary input on the bass amp. Good economizing idea though, most bass amps are pretty passable PA systems in their own right, for smaller gigs.