When you felt yarn that is a ply of wool and superwash wool, one will felt and the other won't -- this means that the felting one will shrink, drawing the other one in as it does so (strangling it, basically ...)
The non-felting ply might pop up here and there, but will stay in place. This is one way to get a faux boucle-effect, since the non-felting one will tend to look bumpy next to the now-shrunken felting one.
It works, and it can look cool -- not sure how it would feel on the sole of the foot, though, if you plan to knit socks.
I've mostly done this with bulky wool and fine silk singles, not with two sock-yarn-weight things. (Silk, like superwash wool, doesn't felt.) Sampling would be relatively easy on the sock machine, or with a handknit swatch. I sampled to make sure my handspun experiment would look okay. Just knit a sample, throw it in the machine, and see if you like the look.
(from a post by me on sockknittingmachinefriends, 3/11/07)