You don't even have to use 2 different balls, just 1 will work. I only ever learned double knit with one yarn in one color - CO an even number of sts and k1 sl 1 across every row. After a few rows you see that it makes a pouch.
This is what the pattern says -
Double knit for 6 rows (*k1, sl1* across 12 times).
Transfer stitches to two dpn (I hold two dpn side by side in my right hand, grabbing the knit stitches with one needle and the slipped stitches with the other needle). Weave in the cast on end. Turn work right side out, pm for knitting in the round.
After you do 6 rows of k1 sl 1, put the slipped sts on one needle and the knit stitches on another, then begin to knit in the round and rearrange the sts so you can use all your needles.
sue- knitting heretic