Isn't BBB signifying Buzzerbeater's Best of the Best ? Therefore, you are the best team in your country. The runner ups aren't.
In a country where two different teams won the Cup and the League, both can pretend to be the best because they can say that they priviligiated one competition to the other, therefore, that's natural that both go to a competition that is to determine who has the best team in the world.
If a single team won both titles, there is no other team that can pretend being the best of the country and deserve to represent it on the international scale.