PO-12 + Piano

I've been using my PO-12 as a drum machine while I practice Piano -- I just put it in SY-0, plug a line from my piano to the PO-12, and then plug my headphones into the PO's audio out. Works like a charm.

The one problem is that I cannot really control their volume independently -- I can change the volume on the piano, but when I change the PO-12's volume it lowers the signal from the audio in alongside the audio it produces. It's hard to turn the drums down and the piano up.

I imagine that this is a solved problem given that chaining POs is a thing, but I could be wrong. Anyone have any great ideas?