You need to get a Dualhead2Go from Matrox which splits a single DVI signal into two screens by emulating a double-sized display.

http://www.matrox.com/graphics/en/products/gxm/dh2go/

Newer Thunderbolt MacBooks can do this by chaining displays.

Having this problem, and I've been told a USB display adapter is the solution, which disappoints me.