Can you? Yes. Should you, hell no. If you have two completely different vocalists, whatever happens to one mic, happens to the other. So, you could be adding too much gain on the opposite speaker, decreasing the low-end on the other that doesn't need it, ect.
And to address your confidence with XLR, it's actually really simple. Pretty much, it is a balanced signal, unlike TRS. It's just overall better and more commonly used, my school has a lot of wireless XLR mics and laviliers.