Happily, we have an official [help-answer for this][1].

>If you have two accounts that you would like to join together, please sign into either account, [visit the contact form][2] and select ‘I need to merge user profiles’. After you contact us, the Stack Exchange Team will reach out to verify that you own both accounts. If we can confirm your ownership, we will initiate a merge.
>
>If you can't remember how to log in, go here and enter the email address you used to create the account you are trying to sign into.


  [1]: https://serverfault.com/help/merging-accounts
  [2]: https://serverfault.com/help/merging-accounts