When creating a morph based head I keep seeing tutorials that refer to DOUBLE BIND. 

So I create a head. I go to LAYER MANAGER and click on the layer with the head. The tutorials show a DOUBLE BIND option in the CONDITION column, but all I ever get is BIND or STRETCH BIND. 

Am I missing a step? What could I be doing wrong that prevents DOUBLE BIND from not appearing? What could be the reason that my only options are BIND and STRETCH BIND?