How to combine/sum a XLR mic input and USB audio input into a dual mono signal?

I have two audio inputs (an XLR mic going into a Focusrite Solo interface and a digital piano input via USB) and need to mix/sum them together into a dual mono signal for FaceTime. So far I've tried creating an audio aggregate device using the Audio MIDI Setup utility, and also tried the audio routing app Loopback, but still haven't found a solution.