Finely chop the kumquats and oranges, combine them, and measure them into a large pot. Add 3 cups of water per each cup of fruit. Let stand in a cool place for 12 hours, or overnight.
Bring the fruit mixture to a boil, reduce heat, and simmer until the rind is very tender. Remove from heat, and measure cooked fruit. Add one cup of sugar to the pot for every cup of the fruit mixture. Mix in the lemon juice, about 1/4 cup.
Return the fruit to the pan, and bring to a boil once again. Boil, stirring occasionally, until the gel stage is reached (the temperature of the marmalade should be between 220 to 222 degrees F, or 105 degrees C when checked with a kitchen thermometer.) Remove from heat, and skim foam from the surface.
Transfer the mixture to sterile jars, leaving 1/2 inch headspace, and seal immediately. Process any unsealed jars in a water bath for 10 minutes. Refrigerate after seal has been broken.