Now this is one that I recently asked around for when a friend of mine was asking if he should upgrade or not. When is it better to have two weaker video cards or one stronger one. As with most computer parts it is very situational when it comes to what setting performs better then another. Both nVidia and ATI ,the top video card producers in my area, have been making it so that using two video cards will split the load of work instead of clashing against each other.
A good way to think about how one video card works compared to two video cards is thinking which is best having one horse pull the carriage or two horses pulling. If you have two horses pulling it will split the weight load and make it so its easier to run at full capacity. But the problem of having two is that it takes more energy to have both running. In many cases it is better to have two weaker video cards then one good video card. If you have the money, lucky to those of you who do, you can have two great video cards working together to speed through every little thing you want to do on the computer.
As of now most video cards are designed to be able to work with another video card. But any video card that retails for ~$80-100 are less compatible and will not get as much benefit as video cards from the newer series. So as I said before, using two video cards is more for people that have the money to get decent video cards, as well as the power supply and other things you would need to support and fully use the power you get from the setting.
If you have any questions of what I put up in this or earlier post, or if you have a question that I haven't yet addressed please leave them in the comments.