You probably have either two male trees or two female trees. You need both sexes for pollination on papayas unless your two big trees were commercially grown and they are hermaphrodites of which if they never produce fruit they aren/t. If your trees are female they will produce small unedible fruit and if your trees are male they won't produce anything but pollen
They are tropical trees. Papayas need a male and female tree but commercial growers have perfected a self-pollinating tree. If grown from seed you will need both sexes
First, how old are the trees? Trees grown from seed need from between 6 - 10 years to become sexually mature. This is why commercial growers either grow trees by grafting onto other rootstock, by cuttings or by air layering. This save a lot of time. There is no way to know just by looking what sex your tree is or if it is self pollinating. If it blooms it is either a female tree or a self pollinating tree and if it doesn't it is a male tree. And no, you cannot give a tree a sex change operation.
