Outnumbered is a simple effect of you assuming you'd have fewer men due to understrength units. There is no inherent penalty to them say "gee, we are missing 40% of our guys compared to nominal strength".

Thus, 2 20 men units is not at any disadvantage compared to 1 40 men unit.

Now if you are arguing it'd be better to have 2 40 men units, then it's a no-brainer. All I'm arguing is that understrength by itself has no effect.

Historically, in long campaigns, units were often at little more than 1/2 their nominal strength in many cases.