Interesting, I've never heard it referred to as "dual-quadcopter". It's a common configuration in the model aircraft hobby where it's referred to as "X8" octocopter. I supposed "X8" doesn't really describe their renderings though, since the airframe doesn't really look like an X.
They call it a dual-quadcopter. That may indicate that the configuration is for control redundancy, as well as increased lift and compactness.