This is illegal and would result in a 15-yard unsportsmanlike conduct penalty -- if the refs caught it. It's also simultaneously amusing and bush league.
The Cowboys apparently complained to refs during the game, but never got the call. It's possible that Stephen Bowen, whom the Redskins signed from Dallas, may have been able to provide information that helped the Skins simulate Romo's cadence.
I'm a firm believer that a team should do everything possible -- within the rules -- to win. Of course, it could be argued (as I've done in basketball games in my youth) that it's not against the rules if the refs don't call it. Still, this sort of thing (if true) gets too close to the "cheating" line.
Enough bad stuff can go wrong in a football season. Don't create the possibility of bad karma by cheating.