I think that you are on the right track.
I think that the most common mistake is to try and rush things.
Just imagine how you would feel if Nakita brought another lady home.
I think that a slow gradual approach will work.
If you pet Nakita more and ignore Phoenix a bit , then Phoenix will look to Nakita for friendship.
There are very few Cats like Scrappy 2, who just made everyone his pal from day 1.
We wish you the best of luck, and hope we see some Dynamic Duo Photos soon.