bnf has certainly proven that it can be done, but surely it is easier to just feed it through once and have both sides scanned simultaneously, right?
I guess it depends upon your scanner. I have the Canon P150 (Image Formula/Scantini) that does duplex scanning. It has a half a$$ed ADF. I'd much rather use the Xerox when scanning many pages, single or double sided. But my main point was that using a simplex scanner to do double sided scanning does not have to be "painful", as bduncan suggested. Using the Xerox with Paperport is pretty much painless.
we need to chip in and get you an even less painless scansnap