I wouldn't recommend layering two different liquid lipsticks, as the nature of the formula means you might not end up with a uniform color on the lips. You could get away with it if you layered them quickly on the lips, one after the other. Another option, if you wanted to create a unique color, would be to mix on the back of your hand or metal palette and then apply with a brush. Your best choices for mixing would be moisturizing liquid lipsticks, as quick-drying formulas will dry before your new formula is made.