You could make two index variables (Index and Skipped) and create a random number between 0-1.
if 0 no pipe will spawn and Skipped+1and Index+1
if 1 pipe spawns and Index+1
spawn pipe at:
X = pipe.width*index and Y = BottemOfLayout-pipe.width*(index-skipped)
EDIT: Sry made a mistake, this one works better