In a lot of WWII movies the soldiers have the liner chin strap up on the front of their helmets and the other strap up on the back of the helmets. Like [here] (http://www.michaelfassbender.org/bobback.jpg). If they're a paratrooper that means they'd have 2 chinstraps and a chin cup. Is this only done in the movies or did the helmets come with two straps, and why not wear them?
The strap across the back of the helmet wasn't really a "chin strap" per se, although it could be used as one when you were just wearing the liner. It provided some benefit to the wearer in that it could be adjusted with small snaps and kept the pot from slipping forward over the eyes when it was worn. The real chin straps were made of a thick fabric and could be worn on either the front or back of the helmet, or not at all. So in the situation that you describe, with the steel pot being used, there really was only one "chin strap" for the wearer to use.
