According to The Wrap, who made sure to confirm things with Marvel first, 2018's Avengers: Infinity War will "exist as one movie" totally separate from the second movie, which is now simply known as Untitled Avengers Sequel. While it's still possible the fight against Thanos will encompass two or more movies, it's expected that each film will be a standalone effort.
So there you go. We know Avengers: Infinity War will open May 4th 2018, followed by the Untitled Avengers Sequel on May 3rd 2019.