Lassig,
Your drawing is exactly what I have. Two 12" siphons with 4" vents. One is about 2" higher than the other, and I don't think the higher one has ever been used. It is absolutely amazing how much water a siphon can move. I am now keeping the vent cap off of the higher siphon just to keep the pond about 2" deeper.
Since my pond is spring fed, there is always at least a good trickle of water flowing through the pipe. Since this water comes off the pond bottom, it would never let the discharge elbow freeze. But living in Ga helps too.
