Usually with long tail cast on you estimate how much yarn you'll need for the cast on and tie a slip knot that goes over the needle. Then you cast on with the two strands of yarn. In order to use two separate strands of yarn, tie a knot near the ends and put that over the needle. Use the two long strands for the cast on. You can see a video here
. Alternatively, you could use a knit or cable cast on for your scarf (also included in the Cast on videos). I like knitting a xcarf this way. It seems to go faster.