Quote Originally Posted by dpneal
Yeah, I suspected that the plastic reels simply wouldn't fit.

As to the "why" of my question ... simple: I own two plastic reels and a plastic tank that can hold one roll of 120 or two rolls of 35mm. I'd like to develop two rolls of 120 at a time, but don't want to spend any more money than I absolutely need to. :-)
Your plastic reel will most probably hold two 120s. Just spool in #1 as far as it will go and let #2 follow just untill it is held completely. Two 120s are about the same length as one 135.