A tank with a single overflow can still run to two zones of a sump. I recommend running the drainline into both the skimmer section and the refugium zone, and I'm often asked how this is accomplished. Using gravity and a few extra fittings, you can direct water to two different zones from a single drain pipe.
With the new set up, I've tried to avoid holding anything back and went "all in" so to speak. So I got busy one night, wiring up this ultra safe system by Aquahub. The way it works: You have two float switches wired in series, which are powered with a 12v DC power supply. When the switches are down, a relay opens the flow to a 110v power cord to supply power a pump that can push top off water from a reservoir into the sump. As the water level rises, the float rises and cuts power to the relay, which stops power to the 110v cord shutting off the feed pump.
A HOB (Hang On Back) overflow or weir is a device used to drain water from the aquarium safely into a sump.
You need to prepare yourself, as this is about 30 seconds of your life that will get your heart beating faster than you'd expect.