Prepare your jars and lids by washing in hot soapy water. Place jars in a water bath canning pot and bring to a boil. Allow jars to simmer while jelly is being prepared. Wash the blueberries and strawberries. Crush them in separate bowls.
Place sugar and 4 cups of each fruit in a large pot. Place on the stove on a low heat. Add the butter to the pot. Stir.Once the sugar has dissolved, increase to high heat and bring to a full rolling boil, stirring constantly.
Stir in pectin once heated to a full rolling boil. Return to a full rolling boil and boil for exactly 1 minute, stirring constantly. Remove the pot from heat and allow to settle for a few minutes. Skim off as much foam as possible with metal spoon and discard the foam.
Fill your jars leaving ¼ of headspace. Wipe the rims of the canning jars with a damp paper towel and put the flat lid on. Screw your lid bands on. Process in a boil water bath for10 minutes.Remove jars and allow to cool on the counter overnight.