Put ripe, firm plums through a food processor using a medium blade.
Combine plums and sugar in a 5 or 6 qt pan. Add raspberries and mix together.
Bring to a boil; reduce heat and simmer until thick, about 40 mins. Stir occasionally. Process according to modern directions from your jars or canner or freeze.
I make several variations of this recipe, depending on how much time I have. I often use a box of pectin with this, just use the directions on the box of pectin for when making plum jam. Also, when using pectin I don't grind the plums, I just pit them and cut them in half. I also add the raspberries at the very end of cooking rather than at the beginning for a fresher raspberry taste.