Managing multiple domains with a single Office 365 P1 domain?

Hi, I’ve migrated my family to Office 365 and am trying to figure out a better way to manage everything.  I have two domains – www.1stdomain.com and www.2nddomain.com.  (made up, of course...!) The first is an Office 365 account I’ve had for a few months and there’s only one account on it – dan@1stdomain.com.  I use that as my primary non-work related email.  I have a brief webpage there and I do use the Sharepoint Online a lot.  The second domain is the family domain – I have a need for three email accounts, but actually created a fourth to manage those mailboxes.  So I have a dan, susan, lisa, & tom mailbox on 2nddomain.com.  I went ahead and signed everybody up for the P1 plan as I’d like to be able to use IM and PC calling for all of us.

I have a few questions though…

* Ideally I’d like to have four accounts instead of five.  I get that each user needs to have an account.  It would be very nice if I could manage dan@1stdomain.com and then add susan, lisa, and tom accounts from the 1stdomain.com admin login - all in the same console.  I see that you can add multiple domains to the control panel but am unsure whether I will be able to buy four licenses under the dan@1stdomain.com domain and then add a susan@2nddomain.com, etc. via the 1stdomain.com home page.  My goal is to simply have a different online identity for my vs. the family.  I "own" both of the domains and have them registered at Network Solutions.