My containers are near as dam empty when I change them.
My inlet hose has a Y piece fitted with two short lengths of hose with stainless tubes attached, these tubes are longer than the height of the container, and find their way to the lowest part of the bottom if you are parked on a hill.
I often only change one container at a time.
If the next job is a big one, and both containers are half full, I will empty one into the other and put another full one on the trolley.