I'd be very concerned about the eventual pressure to make major changes if we told them in parallel just to set them up as foils, as well as changes we'd
have to make to force them into parallel. Any change to a plot has the potential to set off a "butterfly effect" of serious consequences, which may be unanticipated but nonetheless really bad. Example one is the huge cascade of negative consequences and problems caused by postponing all of Beleriand's history until Season 3.
And the Tuor and Turin stories are an example of very delicately put-together stories, where the timing and details were very intricately worked out. Tuor and Turin overlap directly only at Eithel Ivrin, but they overlap indirectly via Gelmir and Arminas, and that overlap is very delicate in its timing and cannot be altered without causing big problems to both stories.
Not to mention the mood whiplash that would result from putting both Tuor and Turin into every episode (
Nargothrond is destroyed! Tuor meets Ulmo and it's awesome! ). Forcing these two stories to be parallel would probably be to the great detriment of both.