Prepare a boiling water bath canner and enough jars to hold 4 1/2 pints.
Combine the cherries, rhubarb, 2 cups of sugar, and calcium water in a large, non-reactive pan. Stir to incorporate the sugar. Once it has started to dissolve, place the pot on the stove over high heat and bring to a boil.
Cook at a boil (reducing the temperature if need be) for 15 to 20 minutes, until the fruit softens.
Once the fruit has started to look jammy, stir the pectin powder into the remaining sugar and whisk it into the cooking fruit.
Cook for another 2 to 3 minutes, until you see the jam starting to thicken. Once you see that, remove the pan from the stove.
Funnel the finished jam into the prepared jars. Wipe the rims, apply the lids and rings, and process in a boiling water bath canner for 10 minutes.
When time is up, remove jars from canner and let them cool on a folded kitchen towel.