Really love and fully agree with the gist of Jeff Patton’s Dual Track Development as well as Desirée Sy’s Adapting Usability Investigations for Agile User-centered Design. Throughout, there are different foci and kinds of work that are needed to evolve great outcomes. Yet, I don’t experience it as a dual track. Also, a dual track seems to split a joint effort into two groups or camps, with the danger of growing into silos and friction.
My image of this important work is much more flow minded. So, more upstream you will find people whose metier it is to continuously and fast discover, create, validate, learn about, and kill lots of ideas, promoting just a few that get fleshed out until they are ready to build by people whose metier it is to develop those ideas into an ever lasting stream of shippable revisions.
Everyone working in or on this flow aim to keep it flowing—No Business Like Flow Business™. Everyone is involved in and accountable for the flow as a whole, yet you spend most of your time in the area of your metier.
Plus, you will need a some people who focus on the big picture and optimise the whole—The Flow Must Go On™.
Also, in flow there are no cycles, yet there is a rhythm:
Drums is the way to start music. To me, rhythm is right up the center of music, and if you start with the drums and get that right, then all the other stuff that requires brain and hard work has a place to go and it all fits much better.