User blog comment:El Alamein/World War Wikia: Sample Battle/@comment-4661256-20141010211500

Also, note that commanders can coordinate joint attacks/defenses of a territory if they have multiple friendly units stationed in the same country. So, if Practice had an ally with him then Hype would have had to take on 2 units at once - and if Hype had an ally with him then Practice would have had to defend against 2 units at once.