Much like the tier sets why didn't blizzard opt to use the same system? An effect for each spec on a single item. This would have cut down the pool giving you more chance at either one you want for main spec or possibly give you a BiS off spec option.
I know traditional legendaries were never multi speced but I feel these are no ordinary legendaries we are beyond that now. We don't pull them from anything significant like illidans glaives we just have them pop up.
I also feel it's not too late for this to be a possibility. The only factor is that people may end up with legendaries in different slots and be down an item where a legendary once was as they didn't expect to fill that place. But they could always be sent a filler much like going from wod into legion with the artifact system.
Would have been much easier to do at the start though.