One GnuPG/PGP key pair, two emails?

If you want to add an user ID to an existing key from the command line (your input is in bold):

  $ gpg --edit-key <key-id>
  gpg> adduid
  Real Name: <name>
  Email address: <email>
  Comment: <comment or Return to none>
  Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? O
  Enter passphrase: <password>
  gpg> uid <uid>
  gpg> trust
  Your decision? 5
  Do you really want to set this key to ultimate trust? (y/N) y
  gpg> save
$ gpg --send-keys <key-id>

Assuming that you use Enigmail for GPG support in Thunderbird, you can simply use the "OpenGPG Security" Tab for each of your identities. You can get there via

Account Preferences -> Manage Identities -> Edit.

Alternatively, you can select from the Menu:

 OpenPGP -> Key Management -> [Right-click on your key] -> Manage User IDs

And then add more IDs to your key.