Depth of Benchwork

Multideck configuration is better suited to narrow benchwork and around the walls arrangement. Even if you have access all way round the layout you will be constantly running back and forth trying to follow your train. Around the walls arrangement allows you to follow the train without mad dashes around the other side of the layout. Around the walls also allows you to support the decks with brackets off the walls.

For hidden support, you will need to support from the wall.

I don't know how to link another thread to this thread, but look at the Jason's blog titled "J&L Railway Blog-The Layout Construction."

Jason built an out building with steel studs, and then bolted his supporting frame work of 2x4 lumber to the studs, and bolted the cross members to the 2x4's.

You need to design the layout for light weight.  The April issue of Railroad Model Craftsman has an article on using Poly Iso Foam-board for light weight, strong bench work tops.  Poly Iso Foam-board is a n insulation material used under roofing.  It is somewhat like foam core from the craft store, but has a fiberglass felt for facing instead of paper.

Use 2x4's for furring strips lagged into your studs on the wall, then put 2x4's in vertically lagged to the furring strips.  Use 2x4 cross members for support of your bench work with "Simpson Strong Ties" steel angles and lag screws through the Simpson Strong Ties, cross members and into the studs, or use 1/2 inch machine screws and nuts with washers under the nuts (the Simpson Strong Tie will function as a washer under the bolt head).  I would also use Lock Tite thread locker to keep the nut from loosening.  

You may need to use braces under the lower level to triangulate the bench work to the false studs you bolted to the furring strips.  If you need some triangular bracing on the upper level, the least intrusive would be to use 1/4 inch steel cables from the studs to the top of the bench-work.  You might be able to use some sort of steel angle bracket above or below the upper bench in the corner of the layout, and then use either the same sort of bracket at the ends or a 1/4 inch thick steel cable at each end going from the top part of the studs to the front corner of the bench work.

Variable Scene Depth

One thing I like to do is have the scene on the lower deck shallower than on the top.  The backdrop structure on the bottom deck can then act as a hidden support for the level above.  I illustrated this idea in a previous blog post:

Note how the backdrop on the lower deck supports the top deck (benchwork only is on the top deck in this shot taken before roadbed installation), eliminating the need for visually intrusive supports.  (I don't mind using visible supports in unscenicked areas, but try to minimize them.)  The shallower scene below also helps eliminate viewers needing to crane their necks to see into the layout.