Yeah, its perfectly logical to separate them.. I mean, it is even the smart choice. Make one a lot harder to find out about and non-existent (anne) and the other one supposedly non-existent and hard to find out about, but you know will become a pirate one day (ace) and when they find out about ace, you would never think there would actually be another one.

Honestly, it is very unlikely such a thing happened, though. But it definitely could happen... It is a manga.. Another thing to point to the "no it can't happen" is that she had to keep ace in her womb longer (didnt she?) .. just think how big she wouldve been with 2 of them.. its like.. even more impossible, right? :-/ but its a manga, so technically it "could" happen...