How would you plumb this?

[BC564]

If you dont want to drain the whole overflow....(power out condition)...then make one of your drains 6 " or whatever you want from the bottom....and the other one longer as previously stated......higher up .....

Put your valve on the lower one and Tee your longer one just below that valve as a emergency drain in case of blockage....

[jvision]

What I started thinking was have the main drain of the Herbie (side w. ball valve) about 2" from the top of the tank, and the back-up about 1" or less higher. Thoughts?

[BC564]

I would leave a bit more room then that....like 4" and 1 "