Those guys who know "The Deluge" mod, will know the "2 factions per team" feature.
Since the US and the British had different allies in the war, its a good option to include that feature too!
A simple click on the "Allies" text will open the troop selection of that allied faction. So 4 factions to choose in ONE battle!
I don't know, why they didn't add that great feature in vanilla NW..