W00t! My double sided boarding pass mistake worked wonders today. In the morning it was folded to show the flight to London. When I arrived there I folded it so it would open on the way back to Amsterdam.
Scanning the bar code was no problem, as they didn't overlap and cause issues with the bar code on the other side shining thru. Nobody complained (not sure whether they evar noticed, have a policy that allows this or have already seen other passengers doing this too).
Next challenge would be to print both on one side and see if the let me board, just because we can, although I think they may have a problem with that (passes are shrunk, more difficult to read, bar code may not scan, it's just plain weird to do something like that, etc.).
As promised two (blurry for authenticity's sake ;-) pictures of the double sided boarding pass (notice "BOARDING PASS" in mirror shining through in the second one):