Pour the cranberry and grape juices into a heavy pot. Add the pectin and stir until dissolved. Bring the juice mixture to a full rolling boil over high heat. Stir in the sugar, and return to a full boil for 1 minute, stirring constantly. Remove from heat, and skim off any foam. Pour jelly into hot, sterilized jars, leaving 1/4 inch headroom. Adjust lids. Process for 10 minutes in a boiling-water bath.