fearsomepirate
Hero
Monsters of the same type go at the same time, not sequentially.
You still resolve actions sequentially, not as though they all happen at precisely the same instant (which would make an absolute mess out of adjudicating reactions). If you want to belabor the point with your DM, he can just say that Pit Fiend 1 launches his fireball 0.00001s before Pit Fiend 2 does. Since the duration is instantaneous, there's no overlap.