Twinned knitting (it also has a scandinavian name that i can't remember how to spell!) is a form of double knitting.
It creates a double layer, 1 sided fabric.
it does something else. look at 'sample' below
1 yarn is V, the other yarn is v (one upper case, one lower)
Look at the center (red V)--see how the stitches above and beside the red V are small (lower case) v's
when you work in twinned knitting (a kind of mesh)
you also make a fabric that is run resistant.
(since the stitches either side an above are worked with a different yarn, if a stitch 'breaks'--it doesn't run (ladder)
this make twinned knitting not only warmer, but more durable.
Amundsen, (the south pole explorer) and his crew all had 'twin knitting' mittens from his mother (he spoke of them in his south pole exploration diary/log..)