Hi. I'm Brian and I will do my best to help you.
You cannot have more than one custom domain within a single subscription. keep in mind also that as of November 30 this year, custom domains are going away. Those with custom domains will still be able to use them, but no new custom domains will be allowed after that and you won't be able to create any new addresses for the existing custom domain. In my opinion, relying on a feature that is going away is taking quite a chance. If they modify the code so that it breaks, why would they invest in any effort to fix it?