Depends on whether or not pumps "stack". 2x Water ladders (2000 l/min) equal the pumping power of a pump trailer. Or if you specifically need one unit with the 4000 l/min
From what i can see this appears to be the case , i had a custom large scale alliance mission we did yesterday and we set it with 250k of water to pump first water ladder turned up pump time was going to be and hr something , as more vehicle's arrived including hvp pods the time reduced dramatically down to about 3mins. so im guessing it does mean they stack