Okay, I suppose tuning the flex is not theoretical, but I think the benefit to a carved turn is. I mean, I can't see how a split in a board can do anything to the flex pattern other than put a kink or hinge in it at the point where the split begins. Therefor, the board is not going to bend into a smooth arc along its entire length. I know the problem of boards kinking in the middle is what Donek sought to remedy with its quadratic sidecut and specially matched flex pattern. Sean has some very interesting side-by-side closeups of boards in mid-carve and the results are telling.
Perhaps this is not a bad thing, in the case of a split tail board like a Burner, where you are supposed to mount them ahead of center - maybe the length of the board ahead of the split handles the job of carving, and the length of the board behind the split handles some other job (quasi-suspension?). Perhaps.
But I just don't see a benefit to putting two kinks in a board. Theoretically, of course!