They won't join.
Hillary won't join because she wants to be out front. If she can't have the #1 position, she won't take any. Besides, she has invested so much political capital in this race, if she doesn't win the nomination, she will never get anything other than her current Senate seat (which may become unattractive for her since it won't be able to be used as a jumping board for a Presidential bid anymore).
Obama won't join because he has thus far portrayed himself as an outsider and standing for change in politics. If he joins with Hillary, he will be tossing that aside, and he will be viewed as more of the same. A smaller, secondary reason is that his wife has told him that this is the only shot he gets- if he doesn't win the White House, there's no second shot, but we'll see if that sticks or not. His rock star appeal will dim the longer he is in the Senate, so if he fails, I am thinking we won't be seeing him trying the same thing again.
I could see either pick up Edwards as a VP candidate, but I highly doubt they'll join forces. Not with the way the Clintons have been acting- they've burned the bridges to Obama.