Peel the mangoes and put them in pieces in a large saucepan. Peel and add the bananas. Pour in the sugar and bring everything to the boil. Let it boil according to the instructions on the preserving sugar pack (usually about 3 minutes). If desired, stir in the cinnamon.
Rinse jars and lids with hot water and place on a clean kitchen towel, do not dry them. Immediately pour the hot jam into the screw jars, keeping the screw rim clean. Immediately close the jars with the lids and briefly turn them upside down.
Tip: Tastes great even without cinnamon.
Can easily be kept in a cold, dark place for several months.