In this instructable I will demonstrate how I installed two complete computer systems in one case. You start with an empty case, an inexpensive, simple mid-tower one will work best. The case must be wide enough to fit the 6.7 inch (170mm) motherboard. The first motherboard is mounted in the usual spot. The second motherboard hangs from 4 long machine screws.
Remove the power supply, you will get at the second motherboard through the hole where the power supply was. If you want optical drives mount them now. If at all possible use SATA drives to hold down cable clutter.
You will need two Mini-ITX motherboards http://www.mini-box.com/Mainboards-Mini-ITX has a good selection of them. I used this board: http://www.mini-box.com/ASUS-J1900I-C. For this board they recommend the picoPSU-90 power supply, http://www.mini-box.com/picoPSU-90. Mini-box also sells the power bricks you will need. You will need two power bricks suitable for your power supplies, this one worked for me:
I have one 3 1/2 drive on one system and a 3 1/3 drive plus a DVD drive on the other, if you use other parts your power needs could vary. A larger power supply is always better than one that is too small.
I would recommend staying with rather low power components, Intel Atom or Celeron boards.
These guidelines worked for me, but every situation is different so you may need to make adjustments.
Step 1: The Front Panel
For the front panel Switches and LEDs I pulled the assembly out of an old case, cut off the switches and soldered on some small push button switches I got at Radio Shack.
Remove the floppy drive bay cover and drill 5mm holes for the LEDs and the right size holes for your switches. My switches fit in a 5/16' hole.
Glue the LEDs in place and mount the switches.
Wrap electrical tape a few places along the length of the wires to minimize cable clutter.
Step 2: Getting Ready to Mount the Top Board
Measure carefully, You will find that you have very little clearance.
Use the motherboard for a template to mark the location of the mounting holes. You want it far enough back so you have clearance to plug your keyboard, monitor, etc in, but leave room so you can get at the optical drive and connect any wires. I marked my holes for the front of the motherboard 8 5/8 inches from the back of the case.
Drill the four holes with a 5/32" drill.
For the mounting hardware I used:
4 - of the thingies in the second photo. They have a 6-32 machine screw 4 inches long. I bought them at Ace hardware.
12 - 6-32 nuts
4 - 1 inch nylon spacers.
8 - 1/2 inch nylon spacers.
Insert the four 4" machine screws and tighten nuts to hold them.
Put a nut on each screw about two inches up.
Turn the case upside down.
Put a 1" and a 1/2" nylon spacer on each screw.
Put the bare motherboard on the screws and look through the hole where the power supply was to adjust the height.
Step 3: Final Assembly
Remove the board and install the RAM, power supply, Front panel wires, and drive cables. Everything that plugs into the board must be plugged in before you install the motherboard. You won't have clearance to plug anything in once the board is in place.
Place the board on the for long machine screws and tighten the nuts.
Now the hardest part is done.
The main motherboard and the drives mount in the conventional manner.
Cut holes in the back of the case for the power connectors.