From Homespun Handknit:
The idea of knitting off both ends of a ball of yarn is an old one, dating back to early Scandinavian knitting. It was virtually lost for years, but has been revived through Elizabeth’s Zimmerman Wool Gathering newsletter and Priscilla A. Gibson-Roberts’ Knitting in the Old Way. Meg has pushed it to its decorative potential, and incorporated a clever and unique “curled tip” finger decrease that her mother invented, or “unvented”, back in 1966.

For the Expert Knitter.

Size: Adult average

Yarn: 840 yd/lb, 12 wraps/in. 4 oz (113 g) of natural white 2 ply wool, and 2 oz (57gm) of light gray.

Casting on for Two-End Knitting
The object is to have single cast-on loops on your needle, but a double strand of yarn to work from. The simplest way to achieve this is to cast on in the usual way, and then join the second strand before beginning to knit.
Meg Swansen’s technique is to use the One Needle Cast-on, holding both strands coming from the ball over the thumb but a single strand of “tail” over the index finger. This gives a sturdy doubled edge, and a couple of loose ends to darn in.
Priscilla A. Gibson-Roberts, on the other hand, casts on using each of the two ends alternately, bringing each end up under the previous one as cast-on loops are made. This technique would be particularly appropriate if you were knitting with two different colors instead of two ends from the same ball.

Fascinating and beautiful mittens that are enchanting to knit.