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.
I was able to demonstrate this configuration in my booth at MACNA 2012. These images capture the method, using a tee fitting, some elbows and a ball valve. Initial plumbing will take an extra 20 minutes of your time but lasts the lifespan of your setup until you opt to upgrade.