Put your 5 empty glass jars into a large pot of water and bring it to a gentle simmer. Leaving them in hot water keeps the glass warm so it doesn't crack when you add hot jam later.

In a separate cooking pot, mix the peaches, blueberries, lemon juice, and pectin powder. Stirring constantly on high heat, bring it to a bubbling boil.

Pour all 5 cups of sugar into the fruit pot at once. Stir well. Let the mixture come to an aggressive, violent boil that you can't stir away. Boil it hard for exactly 1 minute, then turn off the stove.

Take your hot jars out of the water. Pour the hot jam inside, leaving 1/4 inch of empty space at the top. Wipe the glass rims clean with a damp paper towel, put the flat lids on, and screw the metal rings on until snug.

Put your filled, closed jars back into the big pot of boiling water for 10 minutes to lock in the freshness. Remove them and let them cool on a towel for 24 hours.