Okay this is what I was saying... and sorry for the lame "paint" diagram it's just all I could do in 5 min
the 1 is the running water level (your level may be more or less a percentage of the tank)... when power is shut off your tank will be full. The top of the top-up tank is no higher
than the sump/refugium if the Float valve fails (non electric) then it may cause your salinity to drop but it will not over flow... if the power fails and the float is working then it would be the same as no auto top-up was installed... the only way to get an overflow with this system is if the float fails till the water is level with the other one and then starts to work again (preventing back feeding) and then the power goes out. The biggest downside to this setup is you still need to top-up the top-up tank... but it would maintain a stable tank with no change in salinity and the top-ups could be a week between.
P.S. I'm a bit of a scatter brain in the evening I get up early. Sorry for the confusing post.