if you disregard the eyepiece and think of each (2x) barlow as doubling the focal length of the scope it makes a bit more sense
no barlow = 1500mm
first barlow = 3000mm
second = 6000mm
then you can work out the magnification it would give you
i've had some nice views of saturn at 500x through my 12"
absolutely incredible when you get those moments of really good seeing
its not often you get a chance to do it though