Rinse both berries and drain well. Mix with lemon juice and about 1/3 of the preserving sugar and cover and let stand in a cool place for 3 hours.
Then bring to the boil once, pass through a hair sieve and bring the sauce to the boil with the rest of the preserving sugar while stirring. Let it boil for 7 minutes, make a gelling test and then pour into prepared jars and close tightly.
To cool down, turn the jars upside down so that they are securely closed.