How do I set different front pages for domain access sub sites?
You didn't mention versions, so I will assume the most recent: Drupal 7 and Domain Access 7.x-3.9.

The preferred method requires that your 'secondary front page', as you call it, has a different path than your primary front page.

Make sure the Domain Settings module (part of the Domain Access package) is enabled. This will allow you to specify domain-specific configuration options on several admin forms.

Now, when you go to /admin/config/system/site-information, you will see a fieldset at the bottom of the page saying "Domain-specific settings". Choose the domain for which you want a different front page. Under "Default front page" higher up the form, enter the path of your secondary front page and click the save button.