For the peach-pear jam, quarter the pears, peel them and remove the core. Weigh 540 g of pulp, cut into coarse pieces and mix with the lemon juice.
Cut the vanilla bean lengthwise and scrape out the pulp. Mix the vanilla pulp and half of the preserving sugar into the pear pulp and let it sit in the refrigerator, covered, overnight.
The next day, put the peaches in boiling water, then skin them, cut the fruit in half, remove the pits and cut the pulp into small pieces.
Weigh out 880 g of pulp. Bring the marinated pear pulp to a boil with the peaches and the remaining jam sugar in a saucepan and boil for 5 minutes.
Puree the jam with a hand blender and stir in the Grand Marnier. Pour into clean jars and seal immediately.