one thing I can't quite grasp, how common are those type of double bills in America?
they are very common for big event films. Pretty much every cinema in the country here had Infinity War+Endgame double bill on opening night. Same thing goes for Star Wars, lots of MCU sequels, Man of Steel/Batman v Superman, Fast and Furious sequels, jurassic world, deadpool, fantastic beasts, just to name a few in the past couple years.