Good question but I do not know the answer.
What happens if Nakamura or Caruana are finalists in the World Cup?

The next 2 guys I guess.
If it was Topalov & Giri who probably qualified by rating, it would be the next 2 guys by rating.
I'd say FIDE GP has the biggest value besides being the loser of the previous WCh match. Then it's World Cup and then rating. My guess.

World Cup ranks ahead of the Grand Prix. If Caruana or Nakamura finishes in the top two at the World Cup, whoever finished third in Grand Prix (Tomashevsky I think) would qualify.

This is discussed in detail in a blog:
https://chessnumbers.wordpress.com/
along with projections of each player's chances in the WC and reaching the Candidates.
I was looking at the requirements for qualifying for the 2016 candidates tournament. Apparently, the two finalists of the Chess World Cup will qualify. I also noticed that Caruana and Namaura are playing in the world cup. However, they have both already qualified for the 2016 candidates tournament by being the top two guys from 2014-2015 FIDE Grand Prix. So what happens if they also win the Chess World Cup? Who gets those two spots in the candidates tournament?