Need help with overflow plumbing

[sphelps]

If you want a standpipe as the secondary drain a durso can handle more flow in emergency situations because if water gets too high it will start a strong siphon better than a stockman. A stockman or HGB can also work but you have to make sure you have enough space that the rising water will submerge the siphon hole in the cap before the tank overflows.

Having two separate lines is the way to go though, tying them together can actually make things pretty noisy as the flow through the primary will create a vacuum effect in the tied in secondary and suck air through that pipe or just cause a loud echo effect.