So where did Paul go?

Simple answer: It doesn't matter where Paul ended up. His character exists only to provide the tip-off that gets the plot rolling and to compliment both Daniel and the audiences perception of Eli. Just as the distinction between Paul and Eli being left ambiguous provides immediate justification for Daniel to distrust Eli, it drives the audience to feel the same distrust. Whether or not they're the same person is irrelevant, outlining an answer one way or the other does nothing to serve the plot and only harms the intentional leading of audience perception that the ambiguity achieves.

Daniel mentions at the end that Paul is now an oilman with three wells; it would seem odd for him to mention that if they weren't two different people. Eli also mentions another son when he attacks his father.

That said, I agree that creating an identical twin is really only so we can have a set-up for the introduction and something to make Eli feel like more of a failure at the end. Where he is for the bulk of the movie is therefore unimportant.

You're a sexual Disneyland.