I entered a 12 match double xp code at dewxp.com and had the option of keeping it banked or applying and I chose apply. Will I still be able to earn the double xp?
Yep. That’s the only way you can use them, save for gifting them to a friend.
How do you link your gamer tag to apply the matches?
> Yep. That’s the only way you can use them, save for gifting them to a friend.
Alright thanks 
for some odd reason my value went to -12 matches
> > Yep. That’s the only way you can use them, save for gifting them to a friend.
>
> Alright thanks for some odd reason my value went to -12 matches
Don’t worry, that’s just your balance. IE, you redeemed 12 matches worth and “banked” them, then applied them which removed them from your bank. -12 matches just means you have 12 double XP matches waiting for you when Halo 4’s in your disc tray.
> How do you link your gamer tag to apply the matches?
There should be an Xbox Live sign in button at the dashboard