Mix together the pectin and 1 cup of the granulated sugar in a small bowl. Combine it with the raspberries and plums in a medium-sized pot and bring to a rolling boil over medium-high heat. Add the rest of the ingredients and bring to a boil once again, stirring every minute or so and crushing the fruits with the end of a wooden spoon.

Continue to cook the jam for 8 more minutes, or until the mixture has thickened. Pour the jam into the canning jars, secure the lids as tightly as you can, and fully submerge the jars in a boiling water bath for 10 minutes. Remove them from the water bath with a pair of canning tongs and allow the jars to cool to room temperature before handling. Processed correctly, jam kept in a canning jar with an unbroken seal should be good up to a year.