Why would

Because they'd spent their whole lives, presumably, sharing everything. It might never have occurred to them to be exclusive with a wife or a mistress. Notably, we only know one name for them: Alfred. Presumably Ma Bolton gave her other son a name too (and in the Victorian era, I doubt highly she named him Fallon Bolton), but both boys took on the single name and shared it as if there were no distinction between them.

Similarly, we're not meant to have any idea which one of the brothers is the girl's father -- because they don't know and don't care. One of them loved Sarah and the other one didn't, but we are given no textual evidence that one of them slept with Sarah and the other one didn't. If they share everything, and it's heavily implied that they do, then there's no reason why they don't equally consider the child their own.

Nutty, I'm totally with you about the child and the men not caring who the father is. What mystifies me, however, are the scenes where one is thrilled to encounter the wife and child on the street, where one is happy to say he loves Sarah and then another is mis-er-able with her.

I can see sharing everything...but purposely promoting misery? What is the point of that?

I had the same issue, Beej. I mean, I understand that sharing everything is their whole life, etc etc, but in cases where one has a clear preference...why?

Though I suppose you could make the argument that the ruse would fall apart if both of them didn't know everything, down to the most intimate details, about both lives.

I'm with Beej. I don't see what the point of that would be.

Note on the Prestige:

One brother was named Albert and the other Frederick. They smooshed their names together to make Alfred.

Aha! Is that from the book? Is the book good?

Is that from the book? Is the book good?

I wanna know too!

It is from the book. And the book is excellent. I just finished reading it about a week and a half ago--I picked it up by chance long before I knew there was a movie coming out. The style is enchanting.

continued--

I also will not be surprised if there are some things from the book not included in the movie. But from reading the WF I think the reveals are much easier to comprehend in the movie from the book. With the book I walked around for days afterwards, chewing over possibilities... you weren't sure, by the end of it, whether Borden was actually twins or not. It wasn't made explicitly clear, only Occam's Razor kind of clear. Angier's secret was both more straightforward and more diabolical at the same time. You got to the end of it and were simply horrified. I really do reccomend it.

Though I suppose you could make the argument that the ruse would fall apart if both of them didn't know everything, down to the most intimate details, about both lives.

Well, from what little we've seen of them, my assessment was that they just don't know how to do it any other way. Having made that commitment to a single identity, they didn't have any serious coping mechanisms for what would happen if they stopped working together.

Which was the chief thing I was wondering, as I left the theatre. What is Alfred going to do now? Aside from being be severely hampered in his career, and oh by the way officially dead in the eyes of the law, he's lost half of himself. I worried, a bit, about how lonely he would be, going forward.