This has been answered elsewhere if you'd use the search function rather than opening yet another topic.
But, in the interests of sanity, this is what will happen.
Absolutely nothing.
Even if you somehow engineered a collision (a task thats harder than it sounds), there is nothing the game knows to do, to do. The planets are essentially on rails. They can't be slowed down or stopped. You can't snooker them out of orbit. They won't explode. They won't start going binary.
Hell, I'd be surprised if they even noticed each other in passing.