weapon using creatures have a primary "hand" with which they get iterative attacks. They have the same attack penalties that two handed creatures have when using two weapons, mitigated by multi weapon fighting, and perhaps multi dexterity. The only improved/greater MWF I find are in Deities & Demigods, and appear to be for deities only.
So, it appears that Soro's contention that you should just use a single mouth pick weapon, or even just cut off the heads to double attacks, is correct.
Edit: whoops, there I go again, shooting off my mouth. Ettin, for one, get iteratives for each head/hand combo. There may be rules in place that do the same for hydrae. If so, then Jack's argument is the correct one. Because of the hydra's heads.
(I think that, generally, we need to use more words for clarity; that way we know what we're arguing exactly)