I use Crown Royale finishing spray on my two, so I do understand what you're saying about the confusion with two sprays. But, the water bottle is turned to more of a jet type spray than a fine mist spray like the Crown Royale. As long as you use the verbal command along with the spray they will know the difference.....at least my two do.
But if the whistle works - hey, go for it!
LOL...."great-great-great grandfathers rotwiler gene"