Pattern is worked with both yarns held together. For yarn requirements, pattern specified one ball of LaGran Mohair (or 90 yards of another worsted weight) and two skeins of Inca Marl (or 175 yards of another worsted weight).
ERRATA: Note that the body is worked from the top down and not from the bottom up as stated. Knitting is exactly the same - the only difference is that you will sew the base to the BO edge of the body and not the CO edge. (Wiley is working on an errata page for this and other small corrections.)