The use of doubled bronze mallard wing slips on spey flies is nothing new. It has been done since at least the 1870's, if not earlier.
In fact, one can take two slips of bronze mallard (one from a left feather and one from a right feather) that are cut twice as wide as the wing width should be, stroke them so the fibers stand out at more or less a right angle to the short piece of stem you left when you cut the slip, then place one on top of the other so the ends of both feather slips are lined up, and tie them in by slightly tenting with you fingers as you hold them in place for tying in. This will produce a very nice bronze mallard wing.
I couldn't help but notice in McPhail's video that his spey fly had neither the palmered spey hackle the old and new masters use, and he also had his tail and wings be much longer than Knox, Francis, Hale, Kelson, Price-Tannat, and Taverner said they should be. All of these master tyers/authors (with the exception of Knox who was not a tyer, but a great author) talk about spey wings end just before, or slightly beyond the end of the body. They also talk able tails, if used (which is rare with spey flies), are short.
Regards,
Matt
McPhail ties a very lovely fly, but his spey flies due to the proportions and way he tied them, have a very different look from the old spey flies and nearly all the modern masters of the spey fly.
A palmered spey hackled has a very different look than just having a long-fibers hackle wound as a collar hackle. It also takes a bit more time and skill to tie it as the proper spey fly style palmered hackle.