Gmail: Multiple Accounts (Responding)
You can add the option to respond as different email addresses by doing the following
1. Click the 'Settings' icon in the top right (looks like a cog).
2. Choose 'See All Settings'
3. Click the 'Accounts' tab.
4. In the section 'Send mail as:' add the second account.
5. Check the box 'When replying to a message: Reply from the same address the message was sent to'
You should now be able to choose which email when responding/creating emails and replying should use the email the original message was sent to (Useful when you have multiple accounts going to the same Inbox).