Hmm, that question's never come up before...

Yea you can focus on only gaining more portfolios, you can have multiple domains that are the same with the only restriction being that you gain new portfolios with each one. Each one count's as a separate domain, and requires the Gain Domain action. An example being someone having Life (Plants) and then gaining the domain Life (Dragons) or something similar. 

Dang, next time I make a character for this game that should be it's domains. :lol