I have read about some USB devices but they don't give very high resolution. Just do a search in the forum. Mostly people with MBPs do it, and in the past they had Express Card options that offered better resolution than the USB solutions.
In all honesty, the MBA and 9400m are definitely not optimal for such a setup. A Mac mini has built in capability and you can get a refurbished for $499. The newer ones have both mini display port and DVI. So they can run one 24" ACD and one older 23" ACD with DVI.
Good luck.