What is the ruling on sprinkling clothes that have been stained by pre-seminal fluid (madhy) if it has reached the outer trousers, and is it sufficient to sprinkle water on the inner garment that is in contact with the trousers, or must each of them be sprinkled separately?

Madhiy (pre-seminal fluid) is impure and must be washed with water by pouring, and sprinkling is not sufficient. It is enough for you to pour water on the place of the madhiy on both the underwear and the trousers together until you are reasonably certain that the spot has become pure. There is no need to purify each of them separately, and being reasonably certain is sufficient for removing the impurity.