Except Jackson's films didn't flop despite his best intentions.
Beside the point. If they made them back to back and released them 6 months apart, like the halves of the final Harry Potter
movies or Back to the Future Part II
and Part III
, then the second one would still be scheduled and released no matter how poorly the first one did.
Of course, it's thus unlikely that a studio would commit to a 2-part film like that unless it were part of an already-successful franchise. It would be too great a risk to undertake it as the launch of a franchise. (Then again, LotR was quite a risk, wasn't it? I seem to recall there was a lot of trouble finding a studio willing to take the gamble.)