the damage stacking is not the same as Meteor Swarm. here's an example.
you use Meteor Swarm against an opponent with Fire resistance 10. you hit him with the meteor, and then plop another one right next to him, so that he is in both of their areas. because of his fire resistance, he takes 12d6-20 (because the resistance applies seperately for each attack). If you attack an enemy with lightning resistance 10 with my spell, hit with the spear, and then branch off another bolt from that point (so that he is the origin point of the second bolt, he will take 12d6-10 (because the electricity resistance applies once) this only applies with the first bolt, though.