I think you're looking for a Passage of Lines rule, this tricky maneuver was perfected, if you can call it that, way later than the period BLB was written. And a bugger to duplicate on the tabletop. The men of this period could barely chew gum and walk at the same time let alone try to move through each other in an orderly fashion. See page 26, Interpenetration.
One of the ways troops were deployed early in the period was a sort of zigzag line across the front. One in front, one behind, one in front, one behind, etc.. I think this is mentioned in Anatomy of Victory.
For BLB you have to wait until the unit routs through the unit behind and only if there is no other path for them to take. Then see pg 26. Or you could make a house rule to allow a unit to voluntarily break; the save our a$$ rule.