I'm very skeptical of the 1 chain being able to pick up down corn as well as two chains.  I just don't think it is possible for that single chain to pick up the stalk that is laying against the opposite snout as well as a two chain head does.

I actually know of a guy that had a Calmer 20" head with single chains and after the 1st year of using it they sent it back to have the second gathering chain put on every row because they were unhappy with how it worked in down corn.

I'm sure it works fine in standing corn but it seems like almost every year we have some corn that isn't standing real well.