An example, with the largest single unit above the regular stacking limit I could think of without having to edit the OOB.
Moscow '42, most of 7th Panzer's Panzer Regiment.
Setup: 153 Panzer 38(t)'s in T-mode on a major road in one hex with a single (1) Panzer II on a major road two hexes to the west.
First screenshot: The Panzer II blocks movement along the road and the unit can't enter the hex due to the maximum stacking limit. The stack would have a value of 1540. The limit is 1500.
Second screenshot: the selected decombined unit (company) can move along the road and in and through the hex with the Panzer II as the road limit is 350 men, and the total of the two units is 180 men.
Third screenshot: the Panzer II moved off the road. Both units selected at the same time can't end their movement in the same hex due to the maximum hex stacking limit being 1500, so 30 less than would be needed to fit all the tanks into the hex. Though it looks like they can move into the same hex as displayed by the highlighted reachable hexes, in practice one unit will lag behind 1 hex.
This is what happens for road movement with a stack starting in the same hex: as units move one by one, some units will lag behind if the road stacking limit is lower than the value of the selected units, because they pay regular movement points for the terrain to enter a hex with a road that is "occupied" by another unit.
Fourth screenshot: the combined unit can move along the road normally, ignoring both the maximum road stacking limit and the maximum stacking limit in the destination hex, because both only apply to situations involving more than 1 unit.