Re: By all that is holy...DOUBLE NECK XIPHOS???
The 7 string neck is most certainly a longer scale, it's been listed in the specs as 28", and the pictures make it pretty clear that the necks are not the same scale. It's not much of a difference, but the 7 is most definitely longer.
Another possible reason for double neck guitars is to have two very different tunings simultaneously, and if that's why you're doing it, then you might as well go ahead and add an extra string to one of them while you're at it. Having said that, I will also say that the logic behind this makes perfect sense to me for a one-off custom, for example, but I think Ibanez is certifiably insane for making even a semi-production model like this.