Prepare a boiling water bath canner and enough jars to hold 8 half pints.
In a low, wide, non-reactive pot, combine the pitted cherries, chopped apricots, 2 cups sugar, and the calcium water. Stir to combine.
Set pot over high heat and bring to a rolling boil. Reduce heat to medium-high and cook at a vigorous boil for 20 to 25 minutes, until the volume has reduced by at least one-third.
Stir pectin into the remaining sugar and stream it into the cooking jam, using a whisk to stir to help prevent clumps.
Return the contents of the pot to an active boil and cook for an additional 2 minutes and then start looking for signs of thickening (it should be pretty clear as Pomona's pectin sets quickly). Once you see some thickening, remove the pot from the heat.
Funnel the jam into the prepared jars, leaving 1/2 inch headspace. Wipe the rims, apply lids and rings, and process in a boiling water bath canner for 10 minutes.
When the time is up, remove the jars and set them on a folded kitchen towel to cool. When the jars have cooled enough that you can comfortably handle them, check the seals. Sealed jars can be stored at room temperature for up to a year. Any unsealed jars should be refrigerated and used promptly.