Go Forward Together
What Clinton and Obama need to do is hold a joint news conference and announce that each will ask the other to be their Vice-President, but that they are still continuing their campaigns through Pennsylvania and the other remaining states to see who gets to head the ticket. This would keep all the astonishing enthusiasm going, but without the fears of a bitter split after someone wins.
Failing that, when either one does win, they will still need to make the other an offer of the Vice-Presidency anyway, but by then there will be more troubled water under the bridge and a greater chance that hurt feelings will get in the way of accepting. (Refusal to make the offer at all would a political disaster in the general.)
I wish someone would call them and explain this to them. I would myself, but I seem to have misplaced their cell phone numbers....