Instructions
Log in to your Gamertag profile on Xbox Live.
Purchase a map pack from the "Downloadable Content" section of the "Game Library" tab. You can sort the available content by games and find a map pack that you want. Once purchased, the download will immediately begin. A small notification will flash at the bottom of the screen when the map pack has finished downloading.
Sign out of the first Gamertag. To do this, press the "Xbox Media Button" in the center of the controller. A small menu will appear. Press the "X" button, and then press the "A" button on "Yes" to sign out.
Repeat Steps 1-4 with the second Gamertag.