In an 8-quart stainless steel stockpot, combine the strawberries, peaches and lemon juice. Stir in the sugar and add the butter, if using. Cook the mixture over medium-low heat, stirring constantly, until the sugar is completely dissolved. Increase the heat to medium-high and bring to a full rolling boil, stirring constantly.
Stir in the pectin. Return the mixture to a full rolling boil, stirring constantly. Boil for 1 minute, stirring constantly.
Remove the pot from the heat and skim off any foam. Stir in the Grand Marnier. Let the jam cool in the pot for 5 minutes, stirring occasionally.
Ladle the jam into hot jars, leaving 1/4-inch headspace. Remove any air bubbles. Wipe the jar rims and threads with a clean, damp paper towel. Apply hot lids and screw bands.
Process 4-ounce, 8-ounce and pint jars in a water bath canner for 10 minutes. Remove from the water bath canner and let cool for 12 to 24 hours. Check the seals. Store jars in a cool, dry, dark place for up to 1 year.