Can I use 1920x1200 with this dual setup?

"I'll assume Windows, as you didn't specify an OS. Operating systems can't change the limitation of the hardware though, at least not without causing damage.

Your video card should have dual output (DVI, usually). Hook up both monitors and in the display preferences, you'll want to 'Extend desktop to this display' and select one of the monitors as your 'main' display (probably the 28"").

You should be able to select 1920x1200 for the 28"" assuming that is the native resolution for that monitor. However this doesn't magically allow your 20"" monitor to use this resolution, it is limited to its maximum native resolution of 1680x1050.

For example, I'm typing this on a 27"" @ 1920x1200, and my laptop has a 1440x900 display, with some other windows open; I can't make the laptop display go any higher. Nor would I want to, 1920x1200 on a 15"" screen would make my eyes bleed from the smallitude."

John is correct, you can't run your 20" at 1920x1200, but you don't have to mirror the displays you know. They can be set to separate resolutions in a side-by-side configuration.