Inputs only recording on single (L/R) channel via USB

Need some advice, I’m sure Im missing something!

I’m preparing for a live stream over IP video conference - vocal and acoustic piano live using two mics. When monitoring via the UR22C the mix sounds fine with both mono signals sounding on both channels. But when recording a test (using Quicktime) the recorded file records input 1 (vocal) on L channel and input 2 (piano) on R channel.

How do I get it to record both inputs mono L&R?