Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
That style of overflow is also called a self-starting siphon.
As a siphon it works when the sump is below the tank. I do not know if it would work if the tanks are next to each other, just a small difference in elevation.
I have a couple of these working on some tanks. They are finicky, and do not work great. I do not trust them, and check them daily. They may go a week between needing attention, or they have overflowed the tank (siphon running slower than the pump) even when the inlet is clear, and should be no problems. I am really close to drilling the tanks, and getting rid of this style of overflow.
If all you want is a water bridge, then put the tanks at the same level, and just use a U shaped pipe and a fountain pump to move the water from one tank to another. Much safer. Long legs on the U reach to about the mid point of the tank, so even during a nearly 50% water change they are not exposed. I have used a few of these, and did not have any failure of the siphon effect.