DriveLine utilizes two different methods for dimming the screen.
1. Pretty much all video cards support controlling the "gamma" value through software, so this is the easy method. However, it applies to the entire desktop, and not just an individual screen. This is the typical brightness control.
2. Some monitors (most tablets, newer laptops, and some LCDs connected via DVI/HDMI/DP) support controlling the back-light brightness. DriveLine will use this method if the monitor supports it. It *may* be possible to control two of these individually. However, I've never had two of such monitors attached at the same time to play with. And even then, it would require a complete re-thinking of how the brightness control works in DriveLine. Aside from the OSK, DriveLine doesn't pay too much attention to individual screens.
So, the short answer is, "sorry, but probably not".