That's what synchronise arrival is for.
Can someone explain how this works? I want to have leaders gain experience faster so I am going to have a group of single divisions attacking one target. I would like them all to arrive there at the same time though so that I can hold the ground. If they get there individually, they will be beaten back piecemeal, and I would have been better off not attacking in the first place. How can all my attacking units arrive at the conquered province at the same time?