A thought would be ( just of the top of my head have a God bonus too so e.g god of war gives + attack or w/e the relationship bonus boosts that particular god buff boost w/e and if both members of the couple are under the same god it gets and even greater boost......just a thought
Hmmm ok then what about a 2 or 3 stage relationship....eg you get 5 % boost for being a freind with someone ( so long as they are in the same battle grid with you ) 10 % if you are best freinds ( same grid ) and a 15% boost for being in a relationship eg boyfreind girlfreind engaged or married...( perminant )