I don't think you understand how SMTP authentication works and how it all ties together with identities. What you're trying to do is have two Gmail SMTP servers that Thunderbird authenticates with using the same account information, while at the same time behave like two different accounts. That's not going to happen. It's like saying you want to log in to two different/separate Google accounts using the same credentials. If you're exceeding Gmail's outgoing messages limit and you need to send more messages, you should use an SMTP relay service provider and pay for a plan that can carter for your daily outgoing mails.