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.