They dont use siphons to drain actually. They use overflows, which may contain a siphon. The difference being a direct siphon from the tank will drain water at the same rate, regardless of what the return pump is supplying. Think of an overflow like a waterfall on a pond/lake. As the water fills in the pond, the water height rises, and eventually overflows the side. The 'filter' unit is commonly refered to as a sump.
As for what your really asking about, the answer is quite simple. A manifold. There are various ideas for constructing one, but the concept is the same.
As long as the pipes are cut as close as possible to the same length, the water will flow down each evenly, assuming the tank is level.
The same concept can be applied to different materials. You can build a manifold out of PVC pipe alone if you wish. Just have a main pipe of say 2" feed upwards into smaller 3/4" pipes. As long as they are all level it should balance perfectly.
The other way is to install valves on the return right before it enters each tank. By toying with the valves enough, you can get the water output in each tank fairly even. This technique may be better in certain setups where you want one tank to have much less turnover than another. And of course, both setups can be combined as well. A manifold plus a valve on each tank.