If you had multiple singers, all you would need would be something that switched the routing on the mikes. Two is easy, a simple A/B crossover switch would do it. Three or more would be a bit more complex, but still doable. Main thing is, it needs to be just switches, using busses on a board gives you too much opportunity to accidentally send it two voices, which will confuse the hell out of it!

Other thing to watch out for is having similar levels, EQ and tone from all the different singers, or their will be big differences in level, depending on who is singing...