I'm not a lawyer but I think whichever slate wins the most votes goes to the electoral college, but if a party chose another party's slate, that's tantamount to voting for the candidate that the other party's slate committed to so there isn't really much of a crisis there. In your example it would be a roundabout way of voting for Bush so Bush's total votes (i.e. all votes for a Bush-committed slate) would have to be bigger than Kerry's.
I.E. Nader and a greenie doing it with Kerry, though Bush could do it with some conservative 3rd party groups like the constitutional party or something.