ID prints the tiles in consecutive order for each side, I think, before moving to the next side. Even if your printer were accurate enough to get the front-to-back registration correct -- which I can reasonably guarantee it is not -- if you duplex, tile two of side one is always going to fall on the back side of tile one of side one if they print in order.
There's also the issue of the overlap. Even if the correct tiles appear on the correct sides, I think you'll find that they are offset from each other by the amount of overlap.
Thanks for your response. I've been told by a technican at Canon that our machine will print front to back within 1 mm, which is accurate enough for our purposes. The overlap issue is corrected by the use of Tile: Auto Justify. As I said, I can do this in two operations (printing one side, shuffling the paper properly, and then sending the paper through again), but I'd like to find a way to do this in one operation.
As I said, I don't think ID prints tiles one from page 1, then one from page 2. And "within 1 mm" means plus or minus 1 mm, for a total mis-registration potential of 2mm horizontal and 2mm vertical. If that's accurate enough for proofing I wonder why a scaled print wouldn't do as well.