Well a switchbox like that would be easy enough. Two 4P3T switches with knobs, a dozen RCA jacks, and some wire.
I'll even design the circuit and write the manual, for free. Switch positions 1-2-3-4-5-6 are hot1, hot2, hot3, ground1, ground2, ground3 for one channel. The other channel is the same using positions 7-8-9-10-11-12. Duplicate for the output switch. Connect the switches to each other, A-A, B-B, C-C, D-D. OK, my part's done. :^)