I have a Dell Latitude E6400, for which I purchased the following docking station:
The back has two DVI inputs and one VGA input. I already have two monitors hooked up to it using DVI, but it appears I can only use one of my monitors in place of the laptop's viewport rather than in conjunction with it.
Quick and dirty research points to the use of a USB display adapter, but I don't want to use that since I hear those use the computer's CPU to render the display and I can't sacrifice any more CPU than I already am.
Edit: I should have clarified better that what I want to do is use my laptop screen in addition to the two monitors plugged into it. Thus, there will be three total viewports.