It's pretty much impossible for it to happen with 8-23 width divisions since at least 2 of them always will be able to fight ( even at 40 width ), as long as you got decent reinforcement chance. When you to 24 width or above though only a single division can join putting you at the mercy of if the combat tactics will end before your single division breaks (which will eventually fail)
In the defines we can find the following two lines:
COMBAT_OVER_WIDTH_PENALTY = -2, -- over combat width penalty per %.
COMBAT_OVER_WIDTH_PENALTY_MAX = -0.33, -- over combat width max (when you cant join no more).
These together basically means that when you would end up at 0.33/2 = 16.5% or more over width, no additional division can join.
40*1.165 = 46.6 which means 23 width per divisions allows two to join in a 40 width battle, but 24 per division width will not.
Due to how defense & org mechanics works smaller width divisions also become better at defending the worse attack penalties your enemy have ( since there is more org to work through and if their attack can't surpass your defense your fine ).
If I understand your comment correctly in its totality it would seem to me that rather than having all 40 (44) width divisions you might be better off with a combination of different sized units, having smaller units available to fill in the width sizes.