Prepare and sterilize all of the jars and lids for canning. Measure the sugar into a large bowl and set it aside.

Put the chopped rhubarb and peaches into a large heavy stockpot and combine with the fruit pectin (and the butter, if you are using it). Heat the mixture over high heat and cook, stirring continuously, for about 8 minutes. The liquid should be released from the fruit and it should look a little bit soupy and be bubbling constantly.

Add in the sugar all at once and stir quickly to combine. Bring the mixture to a full rolling boil (boils continuously even as you stir). When it reaches a full rolling boil, boil it for one minute exactly. Remove from heat.

Ladle the jar carefully into prepared jars, up to about 1/4 inch below the top. Clean the rims of the jars with a damp cloth. Put the lids on the jars and screw on the rings loosely.

Process your jars according to your canner directions, or for five minutes in a boiling water bath. Remove from the bath and wait for the lids to pop shut. Once they have popped, they are shelf-stable. If for some reason the lid doesn't seal, store the jam in the refrigerator.