Pour the pomegranate juice and the lemon juice in a large pot. Bring to a soft boil over medium heat.
Simmer on low heat for 30-40 minutes. The pomegranate juice needs to reduce to about 1/4 of its volume. Stir occasionally so it doesn't burn.
When you get thick bubbles that are more viscous, you can remove the molasses form the heat. Another way to check if the molasses is ready is to see if it coats the back of a spoon.
Remove from the heat and leave to cool completely. You can transfer the molasses to a bowl to speed up the cooling. Store in an air-tight jar in the cupboard/pantry or refrigerator for up to 1 month (2 months if adding sugar)