It seems both are majority Hutu states and have been controlled by Tutsi Tribes. Why were they split up? Why didn’t they unite at independence?
Rwanda and Burundi were separate kingdoms before the European takeover of the region (albeit with similar ethnic makeups). The Germans incorporated the two Tutsi-ruled, Hutu-majority kingdoms into German East Africa, but didn’t necessarily exert much direct control over the two states. After World War I, Rwanda and Burundi were administered by Belgium as a single League of Nations mandated territory, Ruanda-Urundi, which in practice was governed as part of the Belgian Congo (now the Democratic Republic of the Congo or Congo [Kinshasa] and for a time Zaire). After decolonization the history of the two states diverged; to greatly simplify, the Tutsi minority continued to dominate Burundi, while in Rwanda after independence the Hutu majority took power and began repressing the Tutsi minority. See Burundi, Rwanda, and Ruanda-Urundi.