Complex sentence: It delineates the southern border between Rwanda and the Democratic Republic of the Congo (DRC) and, along with Lake Tanganyika, it forms the border between the DRC and Burundi.
Split into simpler sentences: It delineates the southern border between Rwanda and the Democratic Republic of the Congo (DRC), and between Burundi and the DRC. Along with Lake Tanganyika, it forms the border between the DRC and Burundi.