A simple thing to try, pending an answer from McIntosh, is to connect just one of the inputs, and see what you hear when the switch is set to select the other input. Do that for both inputs, one at a time.
If you hear music that approaches normal volume, it would mean that there is some interconnection between the two signal paths (which I suspect is not the case, given that there is a switch).
If you hear music at a very low volume, it would mean that there is low level crosstalk that is bypassing the switch, for example due to coupling via stray capacitances. That would imply that connecting both inputs at once is a viable approach, in terms of sonics, only if the unused source is not supplying a signal.
Also, if both inputs are connected my instinct is that it would be preferable to avoid changing the switch position unless the amp is turned off. Otherwise you may get thumps through the speakers, or worse if signals are present at one or both inputs.
Regards,
-- Al