Complex sentence: At first, Alan Moore was going to do Action Comics, which was to be a Superman team-up book akin to DC Comics Presents, but Moore proved to be too busy, so Byrne was given that book as well.
Split into simpler sentences: At first, Alan Moore was going to do Action Comics, which was to be a Superman team-up book akin to the DC Comics Presents Superman team-up series. But Moore proved to be too busy, so Byrne was given that book as well.