I just finished my second sweater and I had two strands of yarn going also. I found the long tail cast on worked best for me. I have also seen it called the Twisted German Cast on: good photos
http://knitting.about.com/library/bl...german+cast+on
It is easy to keep loose. I would also suggest to keep your yarn it two seperate ziplock bags. It helps to keep them from twisting up.
best wishes