Simmer pears and plums, covered in a large saucepan with 1/3 cup water until tender, about 10 minutes. Drain; reserve cooking liquid Transfer fruit to a blender. Working in small batches, blend until smooth. Add up to 1/3 cup boiled and cooled water if needed. Pass mixture through a fine sieve for the smoothest texture. Fill 1/4-cup plastic containers or ice cube trays with puree. Use same method for other fruits combinations.