Well, it's common practice to a/b two cameras on one monitor, to match them for color, etc. But you don't see both at the same time, you have two inputs and switch between them (input A and input B, hence the term "A/B" for switching between two cameras).
There is a device made that will multiplex up to four cameras on one monitor, I think they're primarily made for security purposes but some video switchers offer it too.
[quote author=Barry_Green link=board=Links;num=1099060555;start=0#1 date=10/29/04 at 07:42:25]There is a device made that will multiplex up to four cameras on one monitor, I think they're primarily made for security purposes but some video switchers offer it too.[/quote]
They are called quadsplits and are frequently used in TV-productions for monitoring multiple cameras/vtrs/whatever on as few monitors as possible. You can choose to display one of the four sources or all at the same time. However since each picture will only be 1/4 of the screen it's not very useful for focusing and/or color correction.
For use in the field I would go the A/B input route mentioned above and just push the button to change view.
i've been thinking about this lately because i'm getting ready to get a second dvx and my sony 14" monitor has dual inputs with an a/b switch in the front.
i have a friend who does high end security stuff and you can definitely get boxes to do 2 (usually 4) monitors at once but why would you want to? the quality would suck.
if you really want to see both at once i would get a cheap small tv from a discount electronics store like best buy.