Brazzaville and Kinshasa (capital of DR Congo) are the 2 closest capital cities in the world. To cross between them you'll need a visa - and make sure your visa for the country you're leaving is valid for multiple entries, otherwise you'll have to get a new one to come back, too. To get across, the cheap option is the ferry, but it stands until it's full enough to go, and many of the places have minimal shade, so be prepared for a long wait and wear sunscreen! Alternatively, you can ride across in a speedboat as the only passenger - as of late 2007 this cost about GB£ 120.