If you decide to cross from Brazza to Kinshasa, be aware that: 1) you'll need different currency in Kinshasa - Brazza uses the CFA Franc (FCFA) which is valid in various West & Central African countries. Kinshasa uses Congolese francs. 2) If you're planning to come back to Brazza, you should check your entry visa for Congo-B - if it doesn't allow multiple entries, you'll have to pay for another one to get back because even though you're only going a couple of miles, and the 2 cities are almost one, you're leaving the country. 3) It's worth paying someone (trustworthy!) to go to the river port with your passport and get your DRC visa in advance, as it can be a long wait for one if you try to get it when you arrive at the port.