You can use a triple front shifter no problem for a double. When the limit screws are set properly for the front chainrings, it will operate just fine.

Ironically I am looking for a set of STI brake shfiters for a 3x7/3x8/or 3x9 since all I have currently is a couple of sets of 2x8 STI shifters (dura ace and 600). With my weight, I really want to have the bail out bottom chainring if I can't make it up a hill in the middle ring.