I am very glad they are doing a staggered release of this, as if they were going to release them all together I am very sure they would have waited for right before the Hobbit for it (now we get the EE right before the Hobbit).
As for putting everything on one release or keeping it as two separate releases, that also pleases me.
In my family I am the only one who wants the Extended Editions, everyone else just wants the theatrical release. Thus they can get that without the extra cost of having dramatically more material on the disc (i.e. more disc in the set, and thus more expensive).
And I the did put a barebones double edition out, but on DVD the quality wasn't as high as the single releases. While I know Blu-Ray has larger storage, these films are huge (significantly logger then most any film), and while branching would help since they also had to change musical cues in footage that both versions had, it would take even more space. Then you had the vast amount of extras each film had, and yeah to do both versions on one release with all the extras and keeping the quality of the transfer high would make for the set to be of many, many discs. While I would gladly pay for that, most casual fans of the movie wouldn't.