The provinces of Ruanda and Urundi were combined and administered by Belgium as part of the Belgian Congo (1954 to 1960). After the Belgian Congo achieved independence as the Democratic Republic of the Congo, Ruanda and Urundi reverted to their former names of Rwanda and Burundi, and used a common currency issued by a central bank (B.E.R.B) until 1964. After that, each country became a separate entity.
Rwanda-Burundi on Wikipedia