It does seem a sensible use of a couple of channels if you have them spare. For what it is worth, in 1974 I took two Sony spare phon-phono stereo cables from a cassette recorder and made a combiner with some strip block connector, and PVC insulation tape. Just the screens and centres doubled up - twisted together, because back then I could not solder. I then fed my two cassette decks into one input. A simple parallel connection. A year or two later when electronics became my thing, I meant to add the isolating resistors that were visible in every little mixer design - as in the signal came in, went through a resistor, then to the pot, then out from the pot wiper. I looked at my parallel, no component combiner/splitter, and figured it worked perfectly well, so why bother. The cassette decks were not even bothered by the occasional dead short from my bodged up cabling, just no sound. I suppose now if I needed one, I should do the job properly, but that simple combiner/splitter worked so well!
If you don't mind savaging your switch box, you have all the parts to make it work as you want, if you can solder.
