Select the Grande Armee tab in the stack, click "Synch. move", and drag the tab to the desired destination.
All Corps in the same region under that Army HQ should now have duplicate movement orders with the same arrival time.
Note!
The arrival-times on each Corps tab may say something different, but when you run the turn and check the actual arrival times (use the movement filter in the turn report), the move *is* synched to the slowest unit.
Took me a while to figure that one out too.
Obviously this doesn't apply to fixed Corps in the region (e.g. 7th Corps in the Austerlitz scenario, turn #1).
Not sure if it is true should a subordinate commander happen to be inactive though.
Hope that helps.
CheerfullyInsane