The Moderate Voltage sources here are very hot, 24dBU and above, when Mastering.
I never liked the sound of the MPatch attempt at Mono and now I have literally cut it out. I looked at the REW getting started. There is a sentence saying not to use multiple sweeps with 'an acoustic timing reference'.
I still don't think there is any need or advantage to Loopback in your situation, but I will check with John.
BTW why do you want to use multiples? I just use the longest sweep and never encounter S/N issues. I use iMacs and all sorts of gear, without interface often. Y leads are fine for L+R, and I often just disconnect or turn off or down the unwanted speaker.

DD