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 cablescable 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.