Limited knowledge here, but yes they would be plumbed into a single sump. You must remember, marine tanks (the sole reason behind the built in overflows in reef ready tanks), especially reef tanks use a lot more flow so two overflows and two returns would not be considered to much. Your idea of using the other overflow as a WC port and a return intrigues me. It should be possible, you might have to trim the overflow or put a nozzle on the end of the return so you actually get some type of current in the tank, but I don't see how it won't work. You have it so you might as well use it.
Oh congrats on the tank, really looking forward to this build.