... oh yeah.
While it's true that one atom is pretty much identical to any other of the same element, this is where the similarity with the coins ends. The virtual coins in the bank can't interact with each other or the virtual coins in other accounts, but real coins get all over - for instance I might withdraw 50p and get it changed into 10p pieces, but I am still at -50p, whereas in this model I should have a balance of 5(-10).
Also, if you make a withdrawal of £10 and spend £5, you still have -£10 compared to your original bank balance - so in the universe model this would translate to one universe exerting consequences upon the other, but not necessarily suffering them itself.
Atoms arrange themselves into molecules, and objects at a macroscopic level, but the critical element here is that they don't stay that way. Water freezes, ice melts, steam condenses, suns die, rocks break, stallagtites grow, plutonium decays...
So what mechanism keeps both universes "in synch"?
I guess that was a lot to write for one simple question