As an alternative to the method that you described, have you tried stacking two marabou feathers together and then, after tying in the tail, spinning the butt ends in a dubbing loop? I also wonder what the pattern would look like if you tried this technique and used two colors.
I think the best thing about this pattern is the hackle in front of the conehead. It can be adapted to a lot of patterns. I am currently trying some of the mini-cones on soft hackles. Just slide the cone back over the dubbed thorax. I plan to re-publish the directions on my own site in May or June with a few other ways to use this approach.
As far as the marabou goes, I have twisted both together and wrapped, but without using the dubbing loop. I will have to try the loop, as you suggest. My experience is that it is hard to tie down the double-butts without the whole thing slipping a bit. When I tie them this way, I actually lay down some thick nail polish on top of the shank before wrapping.
Good fly Peter. I tie a similar fly for salt species sans cone head. I think it is called a popsicle (?). I think your technique for applying the final collar of marabou is really good and will be giving that a try.