I'd argue that happened when Mockingjay was broke up into 2 films.
But in all honesty, Infinity War will need it. We're talking the possibility of 20+ "main" characters that all need to feel crucial to the story and whats going on. That's not going to work with one film. I mean, the following will all need ample screen time:
Iron Man
Captain America
Thor
Hulk
Black Widow
Hawkeye
Scarlet Witch
Quicksilver
Vision
Star Lord
Gamora
Drax
Groot
Rocket
Captain Marvel
Doctor Strange
Black Panther
Thanos
Inhumans (?)
Ant-Man (?)
Winter Soldier (?)
Flacon (?)
Adam Warlock (?)
War Machine (?)
I mean...that's a lot of people in one film, and the list doesn't even include any of the smaller players.