If the Earth-1 and 2 doppelganger are mirror images of each other, does that mean that their hearts are on the right? That livers are on the left? That everything is flipped for them? If so, that could really work to their advantage... Could you imagine somebody shooting at Black Siren at what they assume is her heart only for her to be like. are you dyslexic or something? and that's the last thing you hear before she kills you lmao
Oh, I’ve already had a whole crisis over the “mirror image” thing, lol. I checked scenes from multiple episodes to see if any of the Wellses had a different dominant hand (they don’t), and tried to figure out how that concept would translate to other earths in the multiverse. Like, how are all doppelgangers mirror images if there are more than two earths? Do things flip every time a new universe is born, so all doppelgangers from odd numbered earths use the same hand while all doppelgangers from even numbered earths use the other one? Or since Earth-1 is at the center of the multiverse, are people from Earth-1 the only ones using a certain hand, while all their doppelgangers across the multiverse use the other? Is the majority of the Earth-2 population lefthanded since the majority on Earth-1 is righthanded? In that case, would they refer to someone’s second-in-command as their left-hand man instead of their right-hand man? Shouldn’t Harry, Reverb, Killer Frost, etc. have flipped faces? Why does Zoom still vibrate his hand through the left side of Reverb’s chest? If Reverb is lefthanded, why does he blast Deadshot and punch Cisco with his right hand?
Like, it’s clearly just something they made up for that one scene and not something they bothered to include any build- or follow-up to anywhere else. So the easiest in-universe explanation is that Black Siren was just calling Cisco and Caitlin’s bluff with her own, making them give themselves away in their panic instead of just being like, “Yeah, I don’t believe you, I think you’re both from this earth,” without being able to prove it.
But personally, I really like the mirror image idea, and I like to think that it is actually a real thing, even though there's only one scene in canon that supports it (and I’ve totally made “left-hand man” an Earth-2 expression in one of my WIPs).