In a saucepan, heat 6 cups red currants with 1/2 cup water over medium-high heat. Crush and stir until skins are softened, about 5 minutes. Press through a fine sieve or food mill to remove seeds. You should have about 2 cups of pulp.
Crush the raspberries with a potato masher or fork, leaving some texture. A few seeds will remain in the finished jam.
In a large, deep stainless steel saucepan, combine red currant pulp, crushed raspberries, and sugar. Bring to a boil over high heat, stirring constantly to dissolve sugar. Boil hard, stirring frequently, until mixture thickens, about 10 minutes.
Remove from heat and test gel using the cold plate test. If gel stage has been reached, skim off foam.
Ladle hot jam into hot jars, leaving 1/4 inch headspace. Remove air bubbles and adjust headspace if necessary. Wipe rims, center lids on jars, and apply bands to fingertip tight.
Place jars in canner, ensuring they are completely covered with water. Bring to a boil and process for 10 minutes. Turn off heat, remove canner lid, and wait 5 minutes before removing jars.
Let jars cool undisturbed for 12 to 24 hours. Check seals before storing. Properly sealed jars keep at room temperature for 12 to 18 months.