When using a triple shifter for a double you have to choose locking out the high position or the low one. I prefer the latter since if you forget and shift into the low position the shift wire merely goes slack while the derailleur stays in the small ring. If you lock out the high position there is the possibility to jam the shifter ratchet by attempting to shift against the high limit stop; unjamming it may be difficult and there is the possibility of damaging the mechanism.