There is not any reason to not have one team consisting of 2 nations.
I can imagine WW2 bf to have for example 15 czechoslovaks with tanks and 15 germans vs 15french with tanks(destined to lose) and 15 polish
Maybe they could even offer part of poland for more tanks :^)
technically they could be 4 teams,but really it would be just 2 with 2kinds of uniform each