The EX2 uses MD driver to do software raid between the two connected drives. (So no, not hardware mirroring)
You can tell the single bay myclouds to create additional MD devices as well, and use USB drives to create an additional array. (See mdadm man pages) It is a little more complicated, because the single bay devices dont want to allow mdadm to create additional MD devices, but you can get the system to create them by abusing the sysfs functionality of the md driver, and echo. 
I made a thread about that some time ago.
To mirror between two myclouds, the EX series use rsync in daemon mode.
The single bay ones respond to rsync over ssh as soon as you turn ssh on.
