I'm guessing you are going to be making a back neck opening for a ways and then knit across all the stitches to join them into one again. Is the problem that little loop that is hanging down in the neck opening on the right side? What kind of cast on are you attempting?
I'm thinking the left side in the picture is fine and the right side is where you are having the problem. Correct?
It may be that having it all on one needle is a problem. Since you have the left side done and it looks okay, why don't you try putting those stitches on some sort of holder for now and go back to the other side and try again.
What I would do rip back until you have the cast on stitches undone and that would put you at the neck edge of the right shoulder (as pictured). Then I would put the needle with the stitches on it into my left hand and with another needle as my working needle use a knitted cast on to add 7 stitches, then work across the 7 new stitches and the rest of the stitches.
I think if you were to put them all back onto the same needle at this point that you would probably have both yarns at the neck edge or both at the side edge. (maybe not) But I don't think that matters at this point. I'd just continue working the side you have on the needles keeping track of how many rows you do. I'm still assuming you are going to join it back into one piece of knitting... Then go to the other side and work it.
You need to end up with the same number of rows on each side, measured from where you put the stitches on your blue yarn holder pictured. I'm not sure where you will be for sure, but when you are ready to join them into one piece again do whatever it takes to have the same number of rows on each side and be able to start working all the way across from one side or the other.
I'm sorry this doesn't seem like a very clear answer, but it is hard for me to explain what I mean.