Let's say I have 2 mics (2 dynamic mics so no phantom power required), they're XLR mics so they take up a XLR slot on my interface (Behringer UMC22). I only have 1 XLR slot and 1 instrument slot (for guitar, bass etc) so only one can be plugged in. I found a converter online that converts XLR cables to instrument cable, can I use this to plug one of those mics into the instrument cable port? Or is it not going to work? They don't require phantom power.
It depends somewhat on whether the line input is balanced or not. If not, a transformer based converter might be okay. If the input is balanced, a passive XLR to TRS cable might work out better. In either case, a line input will likely have less gain than the mic preamp, so unless the source you're recording is loud like a guitar amp, your record level might be quite low. The best solution is a single channel mic preamp.
[Edit] Looking closer I see that the 1/4" input is instrument level. That might be a bit closer to getting the level you need, but I bet it will still be low for many applications.
About the best you can do is use an input transformer such as the Hosa MIT-435. These work as impedance converter and give you about 20dB of gain, depending on the type.