Prepare the milk: drain the soaked almonds and cashews and transfer to a high-speed blender. Add the water and blend for about 30 seconds. Filter the mixture through a nut milk bag and save the pulp for other uses (cookies, bread, etc).
Pour the nut milk into a medium saucepan. Add the sugar, vanilla extract, and salt. Bring to a boil, reduce the heat to a low-medium and let simmer for about 30 minutes, stirring from time to time, until the milk has reduced by about half and has started to thicken. It will thicken even more once cold, so be careful not to reduce it too much or you will end up with a cream consistency.
Remove from heat, transfer to a clean jar and let cool completely before storing in the refrigerator. Use in coffee, lattes, desserts, or as a base for sweet dipping sauces!
Sweetened condensed almond milk will keep for up to 5 days in the refrigerator.