Hi Francis,
The cut you are refering to is called a rabbet cut. It is best done with a table saw and a Dado blade or a power router. It can be done with a rabeting plane, but using a plane on plywood is tuff at best. I have done rabbets with a circular saw and a fence, but not on something as thin as 1/2". You could make a series of cuts and clean the rabbet up with a chisel. But if you don't have the tools do with out the rabbet cut and just but the ends up, glue and screw it. BTW, I used 1/2" plywood and would go to 3/4" if I were to do it again. It's stiffer.