stain over stain

we have a new deck that we have put one coat of stain on ... we do not think that even with the second coat that it is going to be dark enough ..is there any type of color that we can add to the stain that we have left to make it darker ... we purchased a 5 gallon can of stain from Lowes which was enough to do two coats ... so now we have at least 1/2 of it left and we want to darken it ... or should we just "bite the bullet" and purchase a darker color ... AND if we have to do that, what about the one coat that we already have on the deck .... do we just stain over it ... we are SICK, SICK, SICK.. Thank YOu