Several ways of doing what you are trying to achieve.
1. Using your 'Box" Method :
Create first box. Change to vertex mode hilite vertices on one face, the Surface---> Cut away object. Repeat for the opposite face. Now join your boxes at vertices where faces has been removed as you did before.
2. Use Extrude
Create a box. Hilite a face and extrude with *Remove orginal * checked. I believe with V5 you can specify the number of extrustions and the distance apart. (Never tried it though). So you should be able to do it all in one click ... should be much easier than your method.
Regards,
griff