The Managers are associated with the general context I think, so another EntityManager should also "see" the same set of entities.
At least, presuming they are in the same system (of course).
So it's not the EM that actually manages them, but the context that sits behind them (at least as far as tracking the entities in the system).
As far as the key existing in the database, but not necessarily read out into the context, then you won't see the exception until the flush or commit. In other words until the end of the transaction.
It's been a while since I used JPA, and I had forgotten about the differences between persist() and merge(). I rediscovered this helpful article that reassures me I cannot accidentally "overwrite" an existing entity with a new one. (I could still clobber data with a merge, but I'm not worried about that right now.)