What you want to do is select the “Carve” tool and then select the outer square.
When the green arrows pop up, move the mouse so that they only point inward.
Click the left mouse button and then you will have a window with the carve properties. Set the carve width to something larger than the distance between the two squares. This will carve out everything between them.
You can also set the depth of the carve in the properties window.
If you have already selected “Part” for the outside square as well, ESTLCam should automatically do the carve first, then the part cutout. If not, you can always change the machining order manually in the individual properties windows.