Well lets see if we cant get some terminology straight and get this figured out. It should be very simple. Siphons and Durso's dont work together. If for some reason you actually achieve a siphon in a dual durso system you will have floods. The reason being is when you have an actual siphon the water is pulled through the pipe much faster than gravity will ever push it. If the siphon is not intentional like a herby and controlled it wiil break. When it breaks the second overflow will be overwhelmed and cause a flood.
Complex small diameter plumbing arrangements are famous for achieving a siphon when they shouldn't. This makes it nearly impossible to control the second durso. If there is anywhere in your plumbing the water is flow into a y fitting this could be the issue. The single leg of any y should go toward the water source and the two that form the y should go to the distribution points like sump and refugium. If its the other way around and you have little 1" plumbing any back pressure on on one of the legs will create an actual siphon.
Don
Na its not that difficult. If the two pipes going into the y then to the one larger pipe posted a page back are your two durso's then that is most likely the problem. If I'm looking at the picture right and both durso's go to that y they will never equalize.
