Do 2 5V DC wall warts in the same AC outlet have a common DC ground?
Your 2 wall-warts by themselves don't have a common DC ground.  Each one is isolated from AC.  As a result, they are isolated from each-other.

You should be able to power the circuits off two separate wall-warts and have them communicate.  Connect the grounds GNDA and GNDB with a wire.  It doesn't sound that you need to keep the domains galvanically isolated.

An opto-coupler would be useful if you want to keep the domains galvanically isolated.