How do i find out why i'm seeing double only when i look straight and use both eyes at the same time?

First, see your : family medicine or internal medicine doctor at the base's clinic for physical & neurological exams. (S)he'll need history of onset, duration & other symptoms. Seek referral to an ophthalmologist, an MD who specializes in eye & vision problems. Don't just pop into an optometrist's office in a mall. Your primary physician will tell you if you also need to see a neurologist or other subspecialist.