Place the plums, cherries, lemon juice, and sugar in a large bowl. Toss to combine, then set aside at room temperature to macerate for 15 minutes to 1 hour. Place a few small plates in the freezer.

Place 5 clean half-pint jars and lids on a baking sheet. Place in the oven and set to 200˚F.

Transfer the fruit and sugar mixture to a wide, large, heavy pot. Heat over high heat, stirring frequently. After it comes to a full, rolling boil, cook for an additional 10-12 minutes to fully reduce. Remove from heat, then check the set. Spoon a little jam onto one of the plates from the freezer. Return to the freezer for 1 minute, then press your finger into the spoonful of jam. If the surface wrinkles , it's ready. If not, continue to cook for a few more minutes, then check again. When the set is good, add the bourbon or brandy and cook for 1 more minute, stirring well. 

Ladle the hot preserves into the warm half-pint jars, leaving 1/4-inch of headspace. Wipe the jars' rims clean, then gently put on the lids and bands.  Process in a boiling water bath for 10 minutes (count the 10 minutes starting from when the water returns to a boil after adding the jars.) Remove jars and place on a folded towel on the counter to cool. Check the seals by removing the bands from the jars, then holding the jars by their lids. The lids should hold firm. If not, refrigerate that jar and eat promptly.

Store sealed jars in a cool, dark place.