You will need a separate Mixmax account for each email address that you use with MIxmax. Mixmax accounts are automatically created and associated with your email address when you enable Mixmax for that account.
To enable Mixmax on a second account, log into that email account on your Chrome browser. If the Mixmax extension is already installed in Chrome, in the top right corner of Gmail, hover over the greyed-out Mixmax logo. A box will appear - see the screenshot below. Click Enable my productivity.
If the Mixmax extension isn't installed on your computer, you can install it from mixmax.com and follow the prompts to enable Mixmax for your email account. A new Mixmax account will automatically be created.
For more details, check out how to switch between your Mixmax accounts.