In a 6- to 8- quart saucepan, over low to medium heat, cook the raspberries and rhubarb until soft. Lightly smash the fruit with a potato masher. Meanwhile, prepare a boiling water canner. Heat the jars and lids in simmering water until ready for use. Do not boil. Set bands aside.

In a large stock pot, combine 2 cups of sugar with the pectin. Add the lemon juice and sugar-pectin mixture to the berries. Over high heat, bring the mixture to a full rolling boil that cannot be stirred down, stirring frequently with a wooden spoon. Boil for 1 minute. Add the remaining 5 cups sugar immediately, and bring back to a full boil for 1 minute, stirring constantly. Turn the heat to low to keep the jam warm. Skim foam if necessary. Use granulated vegetable defoamer if needed to help eliminate any foam.

Ladle hot jam into hot jars, leaving ½-inch headspace. Wipe the rim with a clean, damp paper towel. Center the lid on the jar. Apply the band until the fit is fingertip tight. Process jars in a boiling water canner for 10 minutes, adjusting for altitude. Remove the jars and allow to cool. I use a pair of rubber-tipped tongs to easily handle hot jars. Check the lids for seal after 24 hours. The lid should not flex up and down when the center is pressed.

Store jars in a cool, dry place.