What could happen? The odds have to be crazy for both of my primary drains to fail no? I know it's best to go worst case and I will ask about 2" again but they said the ghost overflow usually maxes out with 1.5" bulkheads.
Also the aquarium and the sump won't technically be full to capacity which should bring the GPH down. I based my calculations on my 120G sump + 305G tank = 425G but it will be less given the sump is likely to be 3/4 full and tank 2" or so from the top.
If I have the room I'll just add a second emergency drain giving me four 1.5" drains
Anything can happen. Not about odds. Just a suggestion but technially the 1.5 e drain might not handle the flow even if one of your e drains fail. Rough estimate of a gravity fed e drain is 1400 gph. Thats allot of water to be messing around with in your home
The reason why dual herbie was designed with and e drain to match each siphon is because of Murphy 's law


