Multiple casting of Stinking cloud at same time

If multiple casters cast Stinking cloud targeting the same area, will a character who enters that area save multiple times? I know that effect does not stack, but if he has to save multiple times, in case one succeeds one save but fails the other, he would still be nauseated.

If only one Stinking cloud is in effect, what is the DC? Is it the highest DC? What if there are other modifies to one spell that makes them different (e.g. one Stinking cloud with DC 19, and another Stinking cloud with DC 18 but enhanced with Disruptive Spell metamagic)? How about SR (one spell could be enhanced by Piercing Spell metamagic to overcome SR, but has a lower DC)? What happens when it is dispelled using targeted dispel? Or what if using area dispel?