If you're a Twitter user with an old account and a new account, there is no way currently to transfer your Twitter followers from your old account to your new account--that is, unless you're willing to use a little trickery involving one of Twitter's settings. You can use a username-changing feature Twitter offers to start using your new Twitter name while still keeping your original Twitter followers tied to your account.
Sign into the account you want to transfer followers to at Twitter.com.
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Click on your username at the top right-hand portion of the screen and select "Settings."
Type something onto the end of your username to make it different. For example, if your username is BobUser, type "OldAccount" onto the end to change the username. Click the "Save" button.
Click on your username at the top of the screen, then click "Sign out."
Sign back into Twitter using the account you want to transfer your followers from.
Click on your username at the top of the page and select "Settings."
Erase the username in the "Username" field and type in the original username of your other account. To stick with the name from step 3, you'd type "BobUser" into the username field. Because you changed the username of the original account in step 3 to "BobUserOldAccount," the "BobUser" name is available.
Click the "Save" button. The followers you had on the original "BobUser" Twitter account now belong to the new "BobUser" account.