Tutorial:
With High level account, Press A On Xbox Live
With the lower level account, sign in .
With the lower level account, SIGN OUT (Don't Press A to join the lobby)
With the player 2 controller, press A to sign in
With the high level account, press B to sign out
With the player 2 controller, sign into the high level account
With the player 1 controller, sign into the lower level account
- You should now be at the Multiplayer menu.
With the high level account [Player 2 controller], Press A On 'Xbox Live'
With the lower level account (Player 1 controller), Press A to join the Lobby
With the lower level account, then sign out straight away
With the lower level account, press A to sign back in
With the higher level account, (Player 2 controller) press B to sign out
Now just switch the High Level account back to Controller 1, And the Low level account back to Controller 2.
Press Start and the low level account (Player 2 controller) will glitch out and be the same level as the high level account
With the glitching account (Player 2 controller), go back to the Multiplayer Menu, then back to Campaign
With the glitched account, press Start and go to Xbox Live
You will now see that the old lower level account is now the same level as the higher level account