Speaking as a member of the WGAw since 1995, I am disgusted whenever I hear that a studio is doing parallel development on a film. It's a terrible process for the writers, and it reduces the idea of authorship, handing all of the power back to the producers.
It's easy to see why a studio would do it. After all, they're in a rush right now at Warner Bros. to get into the game that Marvel has been winning non-stop for the last few years, and Aquaman is already set to appear in "Batman v Superman: Dawn Of Justice," with Jason Momoa signed to play the character. For Warner Bros., they're trying to get these films up and ready to go as quickly as they can. Having two writers each work on a script to see what ends up working best sounds like a great way to short-cut things.