First look at the bad guys in the second film. You see 75% of the second movie was filmed exactly at the same time as the 1^st film. So the director and producers were able to cast the roles for the second movie while the first movie was being made.

Director Donner got so far behind making the films side by side, that people were wondering if the first film would ever be made. So, production of the second film was halted, and all efforts on put on the first film.