The "Delay" button will do this for you; it works as intended most of the time.
Select the armies you want to sync up using the lasso tool or Ctrl-click, then send them towards the intended destination, then click the delay button. It will default to delaying each stack the amount it needs to be delayed such that each stack arrives at the destination at the same time.
You can also add time if you want them to attack at the same time after waiting a bit.
