It's actually very simple, just have a load resistor, of sufficient wattage, subbed for the speakers when you flip the switch. That way the amps can always see a load when they are not attched to the speakers and there is no chance for accidents happening.
As far as the source goes, you should have a look at the output impedance of the source and the input impedance of the paralleled amp inputs. If it is 10-20X you should be good to go. If not, all is not lost, you can use an output buffer at the output of the source and this should present a high impedance to the source and a sufficiently low output impedance to easily drive the paralleled inputs of the two amps.
Keep in mind that, with some amps, when they are turned off, the input impedance is significantly lowered, so I would do my experiment with one amp off to see if you need the output buffer or not. But they are cheap and fairly transparent so you should have no issues.
As far as the source goes, you should have a look at the output impedance of the source and the input impedance of the paralleled amp inputs. If it is 10-20X you should be good to go. If not, all is not lost, you can use an output buffer at the output of the source and this should present a high impedance to the source and a sufficiently low output impedance to easily drive the paralleled inputs of the two amps.
Keep in mind that, with some amps, when they are turned off, the input impedance is significantly lowered, so I would do my experiment with one amp off to see if you need the output buffer or not. But they are cheap and fairly transparent so you should have no issues.