Aside from what everyone else said (which were good tips, listen to them), there's a point where one hand is waiting for the other to finish a move so they can do a "dual-tech" or whatever you want to call it. The hand should have little purple sparkles around it. Just spam usmash on it while it's like this, it won't fight back until the other hand is finished doing whatever it's doing. You can get a good 100 damage or so with ganondorf on it this way (remember, don't charge your Usmashes, just keep doing uncharged ones over and over, more DPS that way).
And hey, at least it's not the Classic Mode hands on very hard difficulty. There's not a single attack they have that doesn't KO you instantly.