Maybe; you can certainly use polymorph other to turn a guy that's been polymorphed into a stone giant into a rat.
And if you have a bull's strength on you that gives +3 Str & lasts for 3 hours, and one that gives +4 Str but only lasts for 1 hour, you can get one of the spells dispelled & the other will still be in effect. Being polymorphed multiple times seems to be analogous.
Even if they do, though, a targetted dispel magic will get to roll against every polymorph in effect, and thus could get rid of 'em all. Especially if the dispeller is significantly higher level than the polymorpher.