I’m not near my AR, but I thought it was stereo only. The L jack doesn’t default over to mono like the others. Maybe I’m wrong?

Edit: It turns out I was wrong about this. The reason for my confusion was due to an issue I’ve been having with my AR that I am working on with support now. Regarding summing TRS with a Y-cable, I’m still unclear if this would cause a short because you are sending and output into another output and vice versa.