The best logic is whatever makes sense to your end user...
The canonical digital solution: Interlock for your source (assuming you select that first), one buffer for each destination (assuming you select that second) enabled by the destination button and passing the source_FB signals through to the routing signals.
The canonical solution is two analog signals, one to represent the input and another to represent the output. If both analogs are set (determine this with ?ACOMP2 or EQU symbols ANDed together), send a command to the switcher and reset the analogs.
(your description is too vague, this might not be what you want)