2 headunits

The OEM bracket should be enough, assuming it has holes to ISO mount the second unit.

Are you going to hook the audio outputs up of the second unit?

That is NOT a good idea (simply telling him to turn one off before turning the other on).  Eventually he will turn them both on somehow and damage something.

I would suggest using relays on all the outputs that are triggered by the remote wire of one unit.  Whenever that unit it is on it will energize the relays and switch the speakers to that unit.  Something else you could do is use one relay to energize a relay that cuts power to the 2nd unit.  I like the first idea better though, as it will allow you to safely have both units on, and we all like as many lights in our dash as possible!

No, that is not necessarily true.  Even if one amp is on and the other one is off, the amp that is on is also connected to the output stage of the one that is off.  The operating amp is being loaded not only by the speaker but also by the output impedance of the other amp.  This can cause the operating amp to blow.

To use two different amps on one speaker, you must either be able to bridge the amps together (which you cannot do with HU amps) or you need a physical switch between the speaker and the amplifier output.  I like the relay idea, but you couldn't simply trigger them from the remote on lead as that might still allow both to be on at the same time - or both to be forced off.  You need a mechanical selector switch.

And how will that help if you are still trying to power a single set of speakers from different channels of the amplifier?  Same issue as with the HU amps.