Is it true that Angel Beats was originally set for 2 cours before being cut down to 1?
No, Angel Beats! was never supposed to become a 26-episode Anime (2 cours).

Due to the lack of character building, a lot of people have assumed that it was supposed to become a 26 episode anime, but so far all are false speculations. 

I also remember reading the notes about it becoming an anime in either the LN or Manga where he stated something along the lines of not having enough space to tell the story. So they most likely decided on this setup to still give the core points of the story, and getting you into reading the rest. 

One of the sources that mainly instigate this kind of rumors includes small quotes on sites such as TvTropes.org where was said. 


  The reason for this is most likely that the show was originally planned for 26 episodes but was then cut down to 13. Angel beats tropes


Although this might not be the exact starting point of the rumor, they usually start from frequently visited anime related sites, where after they spread to Yahoo, forums, SE.