You could use different connectors.
Like:
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HDMI from PC 1 too monitor 1
VGA/DVI from PC 1 too monitor 2
HDMI from PC 2 too monitor 1
VGA/DVI from PC 2 too monitor 2
Or any combination.
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And just swap between them via the monitor options.
Would be the non-money spend option (if you don't need to buy adaptors that is)

although a bit clunky, it would get the job done, I do the same for one of my monitors as I use it as a TV sometimes.
Or you could buy a switch box to do it for you
I can't find any on overclockers, but just search for 2 way monitor switch box, in google.
For switch boxes:
For what you want to do, you would need two of them, I don't think one single unit exists for what you want to do, someone correct me if I'm wrong
box 1:
Cable from PC 1 into A
Cable from PC 2 into B
Cable from I/O to Monitor 1
box2:
Cable from PC 1 into A
Cable from PC 2 into B
Cable from I/O to Monitor 2
This way you could switch between the different PC's when you turn the switch.
When the switches are at A, both monitors are on PC1.
When the switches are at B, both monitors are on PC2.
You will need adapters obviously though.
There are various devices which can do the same thing, but they are essentially the same thing just different connectors.
Hope this helped you.