Default Site links vs secondary site link - Active Directory Replication

My AD has  6 sites. I have 3 dc in my main site and all other sites have 2.  I have all sites in my default site link with a cost of 15. AD has auto added Rep partner to the first DC in the site back to my main site and the other local  DC in the site. The second DC only has  has AD auto added the other local  DC as a partner.

Example
     Main site server  - W 1, W2, W3
     "O" site   - O1, O2
     "P" Site  - P1, P2

     Secondary site Replication
          Server O1 - partners   W1, O2  ( AD Auto generated)
          Server O2 - partner   O1
I want to add a secondary replication partner for server O2 with P2, just in case Server O1 goes down, so O2 can replicate outside of the site.
How do I do this?