Depending on how flexible your cable is you might be able to make a loop and use the ones you have. Typically I use a longer, very flexible cable and won't change unless I get tired of pulling extra cable through. I don't switch to DPNs. Check the Advanced Videos in the Free Videos section to see how Amy does the loops. Another option is that if you have two circs the same size is to use two of them, I think there's a video for that too. One thing about hats is you can cast on a few more or fewer stitches to make it fit as you wish, you just have to rework the number of sts between decreases. Have fun! BTW I look at hats to find one I want to do and then give up and find some that interest me when I'm looking for someone else. Go figure. Another thought, if your needles are interchangeable you can make two and use a smaller needle tip for the left needle since working in the round means you don't use the left needle to make the stitches. It's actually easier and faster to work that way for me.
I should never overestimate my basic arithmetic skills.