In a 8 to 10-quart saucepan over low to medium heat, cook the raspberries and rhubarb until soft. Lightly mash the fruit with a potato masher and set the pan aside.
Prepare a water bath canner. Heat the jars and lids in simmering water until ready for use. Do not boil. Set the bands aside.
In a glass measuring cup, combine 2 cups of the 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.
Immediately add the remaining sugar and bring back to a full boil for 1 minute, stirring constantly. Turn the heat to low to keep the jam warm, but not cooking it any further. Skim foam if necessary.
Ladle hot jam into hot jars, leaving 1/2-inch headspace. Wipe the rims with a clean, damp paper towel. Center the lids on the jars. Apply the bands until the fit is fingertip tight.
Process jars in the water bath canner for 10 minutes, adjusting for altitude. Remove the jars carefully from the canner and allow to cool on a kitchen towel. As the jars seal, they will often make a pinging sound.
Do not touch the jars for 24 hours. 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. If a jar doesn't seal, store in the refrigerator for up to 8 weeks.