Well, not 100% sure what your customer wants, but here is an idea they could try:
KVM switch with dual monitors, keyboard, mouse, audio...
On each machine, set the computer display settings to use the correct monitor...
So, for PC1, have it default to Monitor1.... for PC2, have it default to Monitor2.
That way, when the switch command is given from the KVM, PC1 would go to monitor 1 only (from what the user sees), and they keyboard and mouse (and audio if desired) will follow. Same thing with PC2 - will go to Monitor2.
Now for the KVM switch not being easy enough. Many KVM switches have buttons on them used to switch between ports.
I personally have a 4 port Raritan USB KVM Switch. It works out really well. It's got the 4 KVM ports as well as a 4 port HUB which can be switched simultaneously between PCs with the KVM portion, or can be switched independently. Besides that, the main unit has buttons on it for switching everything around, so you don't have to use hotkeys on the keyboard if you so wish (I personally prefer the keyboard hotkeys, but to each his own).
This is the Raritan I picked up:
They have enterprise-level models mostly, but this one is more the home/small business level.
And the also have stuff for working over a network, in case the PCs are too far apart to use a typical KVM switch.
Another brand I've used with no real issues is IOGear, thuogh I've only used their simple 2 PC (PS/2) KVM switches, with no box in between. I've not used the multi-featured ones.
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"If to err is human, then I must be some kind of human!" -Me