It will work. Shimano says that the 5703 is triple only because there have been some problems with people using previous generation triple shifters on doubles, using the lower two positions and blocking off the third with the limit screws. When you accidentally try to shift into that non-existent and blocked-off outer position, a small part in the shifter has been known to break (even when using a surprisingly small amount of pressure on the lever). That is why their shifters are now double or triple specific. However, as I said it will work, and to avoid the possibility of breaking something then you set up the cable so that you use the outer two positions of the shifter. You'll then still be able to click the shifter into the non-existent inner position, but the cable will just go very slack and the derailleur won't move.