How to cut exact and identical rectangles with a circular saw?
Try this little trick. 


Cut two rectangles from your plywood an inch or two larger than what you want your finished pieces to be.  Be sure one face of these rectangles are a straight factory edge.  
Now stack the two pieces together and clamp them with the factory edges one on top of the other, nice and flush.  
Scribe a perfectly square line using a builders square perpendicular to the factory edge on both sides separated by the dimension you want the piece to be. 
Now take a perfectly straight piece of wood and clamp it onto the surface exactly at the dimension of the distance of the edge of your blade to the edge of the saw frame. You now have created a guide to run your saw against while making a cut through both pieces of plywood at the same time.  
Repeat this procedure on the other side.  
For the last cut parallel to the factory edge, measure precisely the distance and mark on either side, draw a line between the marks. This line should be square to the end cuts and parallel to the factory edge.  
Now reattach the cutting guide at the blade to frame dimension as you did for the edges and cut both pieces at once again.  


You have now made three cuts, two at a 90 degree to the factory edge and one parallel to the factory edge.  Since both pieces were cut at the same time, they should be identical.