There was a big discussion on the Fixed Gear Mailing List about a year ago regarding this subject (dual drive fixies). Alas, you must be subscribed to the mailing list to view the archives, and I purposely unsubscribed to that list.
http://lists.davintech.ca/mailman/listinfo/fixed-gear

I think the main problem is that it's *really* hard to get everything perfectly aligned... something along the lines of tandems and timing chains? I'm sorta out of my league here, so I'll just post a picture instead. Note that the righthand side of the bike has a *gasp* DERAILLEUR... possibly for chain tension on a freewheel?

EDIT: that's actually a cassette on the righthand side, and it would appear to be a freewheel on the lefthand side. So I have no idea what the hell this picture is for