If you are never going to run both outputs into the card at once you could make a simple
"break out box". Mono jack in from keys or guitar source and feeding both tip and ring of
the 3.5mm stereo jack to put the source dead centre, then just unplug one source and plug
in the other?
Or, have a 6.35mm jack for each source and a switch to flip
between them. Thing is tho', by the time you have bought all the bits you are a fair way
towards buying a Berry X502!