Clearly the purpose of the second lift hill is to allow two cars to duel on either side of the track, whatever Alton Towers say about it being unintentional. As well as that, if there had been no second lift hill, the ride capacity would have been dire unless they'd have put a block brake section in somewhere in the middle which would have been odd anyway.
As to the "two separate coasters" argument; I personally don't see how it makes any difference whether the ride has a second lift hill or a launch. Both are means of generating extra momentum halfway through the ride, they just do it in different ways. There is absolutely no way they could have built it high enough to have enough momentum to get around the 14 inversions by itself via gravity (it would have had to have been 150ft+); even as it is it sometimes struggles to get around each half of the ride (and in fact stalled the other day).