Ford Motor and Rivian Automotive are no longer planning to jointly develop electric vehicles,Ford and Rivian spokespeople confirmed Friday.That actually isn’t a big surprise.Still, the move may leave investors wondering why joint development isn’t a good idea—and what the breakup means for the two auto makers’ stocks.First, a bit of context. Even before the split, Ford in 2020 canceled a program which would have seen one of its vehicles built on a Rivian frame.And this past spring, Ford gave u