I tie a lot of my flies with Wonder Wings.
In the Benchside Reference they are referred to as Reverse Hackle Wings I believe.
They make beautiful wings that retain their width when wet/ginked, which is why I appreciate them so much. They are also easy to make and, as noted, you can make more than one pair from the same two feathers.
Downside (and it's a small one) is that I have found that trout can really chew 'em up (even with dabbing a little head cement on top). I carry a pair of tying scissors - if the top fiber(s) break away from the stem, I just cut it (them) off. The wing may be a little shorter, but it still retains its width.
The Wonder Wing method is a great way of tying split wing paraduns. They are just gorgeous.
Thanks Al & Gretchen
-Steven