You want to present them in whichever order makes it more likely for you to win. If it's easier to prove the service connection for PTSD, make it primary. If your PTSD is more a result of the ringing in your ears, then you'd want PTSD to be secondary to tinnitus. When they are added up, it doesn't make any difference. The VA starts with the higher number and 'combines' it with the next highest number. Go for what wins. Whichever one is secondary won't affect your combined rating.