your design is almost exactly like mine, and it works great.

I made mine using 5- 4'x8' sheets of white board $12.00 each at homedepot and 2x4s I am using a 600w digital ballast that will except MH and HPS bulbs and is alot more efficant than standard ballast's. I then sealed all seams with aluminum tape and the door with weather striping to seal all light leaks. beware of the heat though use 2 inline fans for exahust with odor filtration no real need for input fans just make 1 or 2 more holes in the bottom. Also invest in a remote thermometer with RH sensor so you can monitor the box without opening the door $30.00 to $40.00 at homedepot. I keep mine on the coffee table.

As for 2 plants. I highly recomend the SOG style for a box this size so to maximize your production vs. electricity consumption. I would put 15 to 20 plants in there for 4 weeks vegitive light on 24/7 then 8 weeks flowering 18/6 (18 hours on 6 hours off) this is my way of doing it and many may disagree with this, but with lights off your plant is not growing. The 18/6 is just enough to induce flowering while getting max light for growth during this period.

lol with your first grow and keep us updated.