Can I use two gmail addresses via the same Mail account?

Please help! My brain is fried. I have two email addresses: one is a regular gmail one, the other uses my own domain but it's gmail too. I currently have both set up on Mail on my Mac. I receive and send from both. But I really want to have both working through one account (this is mainly to simplify the very clever Sanebox tool on both my Mac and in the Mailbox app on my iPhone).

If I forward my domain email to my regular gmail then I can receive them all in one fell swoop. Which is fine. But can I then continue to send from both accounts? Is this what they mean by an 'alias'? It's really important that I can send from both.

Sanebox suggested this: http://support.apple.com/kb/TA23485. I'm not sure I really understand it and don't want to risk changing my settings yet in case it all goes wrong.

Thank you very much!