[John Patrick]

Straight answer – you will need 2 transmitters, and 2 receivers. One transmitter on any frequency you like operating in to one receiver which is set to the same frequency. The other transmitter/receiver pair should be set up the same way but on a DIFFERENT frequency from the first pair. The output from each receiver should be fed in to each of the XLR inputs on your Zoom. This setup should work for you.
You can still use the 2 transmitters you have, but you will need to borrow or hire another receiver.
Your receiver can only process 1 transmitter at a time and will take the strongest signal by default. It cannot receive and mix 2 transmitters. When you turn one transmitter off the receiver is then able to receive the second transmitter which is why you are experiencing things the way you are.
All the best.