All kolor products a.f.a.i.k. use only one GPU, so a single graphic card is enough. Not that a dual GPU is a bad thing but it will not bring you anything when using Kolor products. Maybe things will improve in the future but I am doubtfull.
In case of Autopano giga, the stiching programm, only when generating a preview in the editor, the GPU is used. This is likely the time that you sit behind the computer, so important time.
for a formula/system for getting idea what your computer will need for APG
Autopano tour does not use the GPU much a.f.a.i.k
Autopano video does use the GPU a lot when rendering.
In general a computer is more then the speed of a single component. A good ssd or SDD cache can do wonders but first of all you will want RAM, lot's of it