"that makes sense--so how does one build 2x? "

Same way as with the others. You just use shorter spokes and after lacing up all the spokes in one direction like on Sheldon's site just quoted, you twist the hub and lace up a spoke in the opposite direction to hold the twist in place. The only difference at at this stage is the spoke you install crosses only 2 others before going into the rim instead of 3. I have yet to see any quantified numbers on any performance differences between 2x vs. 3x.

"are the trailing spokes over-under or under-over?"

I usually like the pulling spokes going under-over. Meaning they come out from inside the flange and go over the spoke they cross. Seems to cause fewer problems with rubbing the derailleurs under lots of load like hillclimbing. I used to mill my hubs down on a lathe to get the freewheels as close to the spokes a possible for minimal dish and the under-over lacing causes less rubbing when tolerances are tight. Some people mention spoke-damage if the chain gets caught between the spokes & largest cog, but I haven't had any issues with that with proper adjustment.