Add the frozen cherries and the raspberries to a preserving pan. Cover and cook gently for 10 minutes, stirring from time to time, until the juices run and the fruit begins to soften.
Pour the sugar into the pan and heat gently, stirring from time to time, until dissolved. Bring to the boil, then boil rapidly until setting point is reached (5–10 minutes). Skim with a draining spoon or stir in butter if needed.
Leave to cool for 10 minutes so that the cherries don't rise in the jars, then ladle into warm, dry jars, filling to the very top. Cover with screw-top lids, or with waxed discs and cellophane tops secured with elastic bands. Label and leave to cool.