I have an on-board video card on my motherboard, I also have a separate video card and at the moment I use the latter. I'm going to buy second monitor, can I use one video card with each monitor? Or is there no way and I must switch both monitors to one video card?
Recent Intel motherboards (i7-920, i7-2600K) that I've used automatically disable the onboard graphics outputs when a dedicated GPU is connected to the system. I was able to re-enable it by setting the BIOS option
In my experience, run both displays off of a single card if you can. Pretty much any discrete card worth spending money on will support at least 2 independent displays, and it can save you a lot of headaches about hardware accelerated video.
Generally, if there are two or more video ports on a graphics card, you can run 2 monitors off of the single card.
More often than not, though, when you plug in a new graphics card, any on-board graphics are disabled.
As stated in the comments, though, knowing what chipset and what graphics card your are using will assist in determining a concrete