Will a double 2x4 wall 24" on center with an attic truss above meet building code? If not what does one have to do in order to get it passed? Two 2x4 have to be better than one 2x6.

The inner 2x4 wall won't be load bearing unless your roof trusses are specifically designed to transfer part of the load from the top cord to the bottom cord right over the inner 2x4 wall. It can physically be done - you just need to work with your truss designer on the details. How much space do you intend to put between the inner and outer 2x4 walls?

My guess would be that a truss like this (plywood or OSB gussets) would provide a wide area to transfer load to two exterior walls.

Most modern truss connector plates aren't nearly as large as the ones shown in the link above. Those are more like homemade trusses with plywood gussets.

If someone is going to the trouble of double exterior walls for extra insulation, then they also should be considering raised heel trusses to allow more insulation right over the top of the wall. With a raised heel truss, the roof load is even more concentrated right over the outer wall unless an extra vertical reinforcement is placed right over the inner wall.

Example of raised heel trusses: http://www.builderonline.com/roofing/raised-heel-trusses.aspx

I am building a similar wall on my addition. The exterior wall is 2x4 with OSB and trusses on the top 16 on cneter. My interior wall is 2x3 24inches on center. I am also considering osb under the drywall to make the wall more rigid and to prevent the dense pack cellulose from bulging any drywall. I will also not have to be as picky when hanging stuff. I am sure it is plenty sturdy since the 2x4 outer wall alone passed inspection to hold the trusses.

I am curious about your decision to use dense pack cellulose vs damp sprayed. What are the tradeoffs for new construction?

For me, mainly because I can do it myself. I buy 200 bags of cellulose at Lowes and I get the blower for 24 hours free. I can fill the walls from bottom to top and hopefully very little time. When it settles I can pop a board off in the attic and add more.