I don't know about your amp specifically, but on most that have both single-ended and balanced inputs, the "switch" between them is not actually an input selector in the preamp sense, but is only there to prevent hum which could otherwise be induced (in place of input shorting plugs). If the switch doesn't defeat your presently-used single-ended inputs when flipped over to balanced, this is why, and both sets of inputs are always 'live'. Assuming that's the case, then you wouldn't want to connect two sources to both sets of amp inputs simultaneously because of the parallel impedance.