[Illuminated One]

Ok, a test with 3 nations

Arco (nation nbr 0)
Ermor (1)
Lanka (something highish)

All using vastnesses thanks through modding.

4 provinces
120 and 121
90 and 170

First trial (9 times) Arco and Ermor moving armies from 120 to 121 or vice versa.
The armies never miss each other. Sometimes Arco is the defender, sometimes Ermor.

Second trial (about 20 times) Arco and Lanka oving armies from 120 to 121 or vice versa.
The armies practically always miss each other. When they fight sometimes Arco is the defender, sometimes Lanka.

Third trial (7 times) Arco and Ermor moving armies from 90 to 170 or vice versa.
The armies sometimes miss each other. When they fight Arco is always the defender.