Yes, this can be done. Shift down to the smallest chainring, flip the small shifter a few extra times to make sure that all of the cable is out of the shifter.
Release the cable from the derailleur. With the double crankset on the bike adjust the front derailleur following all of the instructions found at http://www.parktool.com. Do all of the steps in proper sequence leaving nothing out.
The triple derailleur should work OK with the double crankset but not as well as a double derailleur. It is always best to use derailleurs and cranksets designed for each other but there is quite a bit of tolerance for mismatched components. The triple derailleur is wider than the double.

Al