So are you just trying to do a long tail cast on holding two colors together? And then you knit one row and this just appears? Have you done the long tail cast on quite a bit before with only one strand and never had this happen? The two strands shouldn't make any difference. I don't know what is happening. I've never seen anything quite like that.
If we could see you cast on and knit your row I'm sure we could catch the problem, but it is harder this way.
Knitting can really throw you weird curves sometimes. I'm sorry it is giving you so much frustration.
All I can say is that if you are experienced with the long tail cast on and carefully do everything just as you would with one strand, I don't see any reason that would happen.