Peel the rhubarb and then cut into pieces approx. 2 cm long. Peel the bananas and cut into small pieces. Then simmer the chopped rhubarb and the bananas over medium heat to a puree. Stir in the preserving sugar and cook for 3 minutes while stirring. Pour the jam into jars. Then turn it upside down for five minutes, then turn it over and let it cool down.