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. :-)
Thanks for the replies! I think that I'll need to look at that Patterson plastic tank that will hold two rolls of 120 instead of a cheaper metal tank.