Originally Posted by https://www.quora.com/Is-it-possible-to-merge-two-YouTube-accounts
Instructions
Log in to YouTube using your Google account credentials. Click "Account Settings," then click "Manage Account."
Select "Change Google Accounts" from the "Change Which Google Account is Linked to This Account" pane. Enter the image verification string, then click "Unlink My Accounts" to unlink the YouTube account from your existing account.
Input the username and password for the new Google account with which you want to link your YouTube account, enter the image verification string, and click "Link My Accounts."
Doing so merges content from the YouTube channels together, allowing subscribers and friends from both channels to view all the content via the same interface.