Ben,
I hate to be the one to come in and burst your bubble, but from my point of view, that just ain't gonna hold water!
First off....in the space you have on the bottom module, you would be hard-pressed to do a decent loop, let alone a loop, yard and industries. Secondly, a helix is massive....to gain 2 feet, you'll need at least a 4x4 foot helix, with rather steep grades! Thirdly, i'm 6'2" so 6' is just about eye-level for me. In my neck of the woods, i'm abnormally tall...most people that come to train shows, in my experiance, have been less then 5'8" tall....how are they supposed to see the top shelf? Another point...do you have a full-size van, UHaul or F350 to haul these modules around in? Not only are they bulky, they're heavy and awkward....and they're going to take up ALOT of space. I've got one 3x4 module, and i need to rent a van to move it without damage....the full-size family sedan wouldn't hold it.
Yet another point i wish to make is....i remember you saying in an earlier post that you wanted to model the whole UP railway. And you sound in this thread as though you want to model a lot as well. Now, either you don't get out much, you didn't do any background research, or you REALLY believe in massive selective compression, but you've got a slight problem....you've got two rather small (relitivally speaking) modules, and you want to model a railway that currently owns and operates over 36000 miles of track.
You should also be warned that Digitrax does NOT use phone cord and phone jacks for the loconet network....it uses special Cat5 6-conductor wire and RJ12 Connectors....look like phone equipment, but they're nothing NEAR the same when it comes to operation. I would suggest a GOOD read-through of the Digitrax manuals, and a good sit-down to think about what you have, and what you want to get done.
I hate to come in here and sound all negative, but i feel it needs to be said....it can't be done. A double-level module isn't a safe or sane idea, in my opinion, and your trying to cram too much into the given space. Sit down, think it through, and maybe do a home layout instead of a double-deck module, and do a module to take to the shows and show off there. Maybe only do one of the two....but in my opinion and my experiance, it ain't gonna hold water.
Andrew Reid