It could have, or not have as much output as 2 separate 10's, depends on what kind of subs. And its not the same as 2 separate tens, instead of using 1 larger driver, they use 2 smaller size ones to get the benefit of a larger surface area and use a smaller box, and gain some speed.
My impression is that 1 sub with 2 10's is better than 2 separate 10's, and the reason is that if they are in one box, you *can* get deeper extension because the manufacturer probably eq'ed them such that in addition to greater output, you get lower extension due to the great area.
In the 2 separate sub case, you get greater output, but no benefit to lower extension because they are both singularly limited to what they each individually are.
I can't explain it right, but maybe you get the idea. For low end extension anyway.