Arrange two of the 21-inch beams with both 30-inch beams to form a rectangular frame with the shorter beams abutting the inside faces of the longer with the 2 inch edges on the work surface. Screw together with two screws per corner, driven through the faces of the 30-inch beams into the ends of the 21-inch beams.
Lay the 30-inch by 24-inch plywood sheet on top of the frame. Screw in place with one screw at each corner.
Set a 30-inch post in one corner of the frame. Screw in place with two screws driven through the plywood and into the end of the post. Reinforce with one screw through each beam and into the sides of the post.
Repeat step three to mount the other three 30-inch posts in the other three corners.
Repeat step one using the remaining four beams, building a slightly smaller rectangular frame. Mount the remaining plywood sheet to the top of the frame.
Repeat steps three and four to mount the four shorter posts in the four corners of the smaller rectangular frame.
Sand all faces of your finished desks.
Set the larger of the two desks in position in the bedroom. Tuck the smaller desk in beneath it. You can pull it out whenever you need to.