There's a positive head from the upper to lower barrels giving a feed i.e. decent pressure from higher to lower. If the stillage of the upper were also angled it would drain further than if level.
An interesting photo but not something I would faff about with myself, I use a similar regime to John in post 6; swap aquarolls after evening meal giving enough for the next day.
Thank you for your thoughts, but that could not be the case, becasue the barrels both need to be able to draw air in as the water empties out, so the upper most caps have to be able to allow air in. As such the connection between the barrels (the blue pipe) will always allow the water levels to equalise in both barrels to achieve the same horizontal level. That means the lower barrel will contain more water than the upper.
If two full barrels were set up as shown, the upper barrel will discharge into the lower barrell causing it to over flow through the pumps cap, until the water level in the upper tank exactly matches the top of the lower tank, i.e. the water levels have equalised to the same horizontal.
Edit - addition of image
Simply the physics of hydrodynamics, water in all connect vented vessels exposed to the same air pressure will settle to the same horizontal plane.
There is no advantage in raising or lowering one barrell above the other.