I'm not sure how big a fan I am of having 2 ultimates.
One of the biggest strengths of OW is it's clarity. You have 23 characters, and you basically know what each of them is capable of. I don't think Symmetra's 2 ults will cause too much confusions, but I worry about what kind of precedent this sets. I don't want the game to change into something where I have no way to telling what kind of attack an enemy is going to perform on me.
It doesn't set any kind of precedence. Every character in overwatch has something unique about them. I would say symmetra being about to pick which ult to put down is no more unintuitive than Lucio's cross-fade.