I did this with my first sock.
What you need to do is cast on all of your stitches and then divide them among your needles.
When you join, postion the needles so that the first cast on stitch is on the left needle close to you and the last cast-on stitch is on the needle in your right hand close to you (the working yarn will be coming off of the right needle). The back needle will contain the middle stitches.
The needles will look like this:
Now join into the left needle. Your stitches will then be stitched on the outside.