yeah, it can be done, but it's going to suck.
like Nicofiery said you can get a pair of adapters
XLR Female To 1/4-inch Male Connector and a Y splitter
Y Cable - 1/4 in. M to 2 1/4 in. F
thing is - any time i've tried running multiple devices into a single amp using the Y cable one side was always VERY dominant over the other. using it to squeeze two elements into one output is actually the opposite of what it's intended to do. it might be fine for lead and backup vocals so long as there isn't much variance in what's supposed to be heard.
though it's more likely to end up sounding like a cheap homemade chorus / reverb effect and if you need distinct interplay between the voices i think you're in trouble with this plan.
it never hurts to give it a try though. at the same time it's usually not 'that' expensive to rent a PA for a weekend (the initial deposit is the most painful $ part). and you're entire band will sound way better.