The premise does seem stupid. I've never know twins that the people who knew them couldn't tell apart from 50 yards.
Like the description says, no one in Siobhan's life supposedly knows Bridget exists. It makes a bit more sense that way.
Also, I don't think you can knock something because it's something one hasn't come across. There are plenty of identical twins that you wouldn't be able to tell from inches away, let alone yards. With alot of twins, how they act is how you can tell them apart.
I also hope that this show doesn't get knocked by critics the way Fringe has this past season. In the sense that, i've read many people bashing the Peter's actions for not knowing that he was dating woman that was actual a another universes version of the woman he knows. For one, I don't see how one can knock someone for not knowing something we the audience knows. It's easy to point out what Peter should have noticed in terms of differences (and he did notice them for the most part) but he told himself that it was her being in a relationship that changed her. Really, if you have no reason to doubt who the person is, there is no reason for picking apart that person for not figuring it out.
In the case of the Ringer, the husband doesn't even know the twin exists. Why would he think the twin is anyone other then the woman he loves? Even if she is different in some ways, he has no knowledge of a twin so him assuming something like that would be extremely odd in my opinion. Also, why, would he know his wife has a twin if his wife or no one else told him? if he doesn't know, he doesn't know. I don't find it odd at all.
CW has shown through Nikita that it can handle more adult shows on their network. A pairing with Nikita would have been awesome but tuesdays are a dead night for me at this point so i welcome it to tuesdays.