True, there are 2 currencies in Cuba, the so-called convertible peso which you will get at money exchanges and the Cuban peso used by locals. US dollars as far as I know are subject to a 10% discount unless that has changed recently. Tip in convertible pesos if you feel the need, remember the Cuban peso (CUP) is about 24:1 to the convertible (CUC) peso. Both are closed currencies, meaning you cannot get them outside of Cuba.
Trip Advisor guru Bellagio wrote a great article here on Cuban money and credit cards.
https://www.tripadvisor.co.uk/Travel-g147270-c69372/Cuba:Caribbean:Currency.And.Credit.Cards.html