Just figured I'd post a follow-up to this if you're looking for an approach, and possibly even some measurements.
Here's my quad stack double cabinet.
http://festoolownersgroup.com/festool-jigs-tool-enhancements/home-made-sysports/msg139419/#msg139419
And specs for what I built:
Part Dimensions (mm) Quantity
Top/Bottom 922 L x 460 W 4
Sides/Ctr 896 L x 460 W 6
Rear panels 922 L x 936 W 2
Shelves 407 W x 455 L 18
Doors 932 L x 461 W 4
I used 20mm plywood for the sides and top.
I used 1/2" plywood for the shelves.
18" full extension drawer slides
The shelves have a little bit of extra width, which can be made wider or narrower, depending on how the whole construct comes together. Make those last!
The quantify of shelves depends on what you're going to have in that stack.
I put together a spreadsheet with various stack options, so I could mix and match for maximum capacity, while staying at/below 40" total. For this, I did it in imperial measurements, since I have an easier time thinking through spacings and such in that....
Example:
Config #3 (inches)
Top Carcasse 0.75
Top space 0.25
C12 Systainer 1 4.25
Shelf 0.5
Bottom space 0.25
TDK Systainer 1 4.25
Shelf 0.5
Bottom space 0.25
Screws1 Systainer 1 4.25
Shelf 0.5
Bottom space 0.25
Screws2 Systainer 1 4.25
Shelf 0.5
Bottom space 0.25
MFK Systainer 2 6.25
Shelf 0.5
Bottom space 0.25
RAS Systainer 2 6.25
shelf 0.5
Bottom space 0.25
Bottom Carcasse 0.75
Casters 4
TOTAL height 39.75