Gillard and Rudd cannot coexist in the same caucus. Their factions are too entrenched and would sooner spite the other than close ranks against the Liberals. Self-reinforcing spiral, really, when you're resigned to getting caned you want to have as much post-government leverage as possible. Which means leaks and more party breakdowns.
It's basically 2004, except the factional insurgency now has a cherubic, bespectacled face.