With two masons, you have two votes, and two people confirming each other. So that's information to the town, and that's it.
But with a hydra, the two heads are only confirmed 'whatever' to each other, not to the rest of town. Ok, so there goes alignment. Then, they only have one vote. Also consider them having to possibly argue over their scumpicks to get what they want.
So really you only get the communication aspect of the mason role. Town recognizes that. Does this help the slot IMMENSELY beforehand? I don't think so. I think it's neutral. As neutral as a neighbor role. I think the more effort you put into the hydra, the more can possibly come out. (If you are sync with your other head) Once you take advantage of the hydra, the advantage is clearly there. But if you slack off, there won't be any 'advantage' to your exposal as if you were some mason, because you aren't a mason. (Again, neighbor) You'll come back to it and realize it's still sitting there untouched like you left it.
So it's like...
Oh. MY. GOD.
I just thought of a cool idea.
Some masonry/neighbors that can only have ONE vote together. Can that work?
Also, I love playing in hydras, because masonry is my favorite role. As a mod I'd prefer to keep my games non-hydra, but I'm biased because I love to play in them. It's hard to find that happy balance. Maybe what flay said could work, every other game NO HYDRAS's or something to fulfill that niche. Or hydra limits.