He was talking about buying a really cheap, like $30 card to run the 2nd monitor.
As far as that goes, it will lower the temps at idle. At load, it won't make much of a difference, other than it already has a head start on getting to it's load temps. The reason it's hot when running 2 monitors, is not because it's doing a lot more work, but because the clocks don't power down as low as when you only have 1 monitor. Once you are in 3d mode, it will be the same.