For the raspberry-rhubarb jam, cut the peeled rhubarb into small pieces. Mix the rhubarb pieces with the preserving sugar and allow to infuse.
Briefly rinse the raspberries, pat dry and mash a little with a fork. Bring rhubarb to a boil, add raspberries.
Boil for 8 minutes until bubbling, add cognac and pour into clean jars and seal tightly.