Ed Balls could pull out in favour of David Miliband, but Milly could only make him Shadow Chancellor if he were elected to the Shadow Cabinet. There is no guarantee of that.
How priceless, if neither of them were to be elected to the list of those to whom Ed Miliband was to allocate portfolios. They would probably both announce that they were leaving Parliament at the next Election, as Tony Blair had been about to do until John Smith died.
As much as anything else, that would be one in the eye of the BBC, which has been campaigning for David Miliband ever since Blair announced that he was standing down.