Pour canned coconut milk and cashew milk in a large saucepan. Add curry paste, soy sauce, lime, and cayenne (optional). Gently stir until evenly combined. Add a pinch of coconut sugar (about 1 tsp) to neutralize the acidity. Adjust spice level by adding more cayenne or curry paste as needed. Allow mixture to come to a simmer on medium-high heat, then simmer on low for 10-15 minutes. Feel free to add more liquid if the sauce gets too thick.