Doublethink means the power of holding two contradictory beliefs in one’s mind simultaneously, and accepting both of them.
George Orwell, “1984”
The test of a first-rate intelligence is the ability to hold two opposed ideas in mind at the same time and still retain the ability to function.
F. Scott Fitzgerald
ergo
Doublethink is the test of a first-rate intelligence.
Q.E.D.
Unapologetics 
But ‘ideas’ are not ‘beliefs’, are they?
All beliefs are ideas, but people do not believe all ideas, of course. I think, that’s a technicality. I think, Fitzgerald meant “holding opposed ideas” as “believing in opposed ideas”, depending on circumstances and context. Like physicists may consider an electron to be a particle or a wave depending on which properties they describe or considering a “hole” in semiconductors to have properties of a particle. Or for a scientist who believes in God to be a skeptic and a materialist in scientific research.
I disagree, I think Fitzgerald actually meant the opposite. I think he was talking about being able to objectively consider and examine two opposing ideas simultaneously, from a neutral standpoint. Whereas Orwell seemed to be referring to people who actually personally believe in two opposing ideas with the concept of doublethink. One can hold two opposing ideas in mind without believing them, but one can not believe two opposing thoughts and still be objective. Doublethink may be closer to the opposite of first rate intelligence, not the same thing.
I didn’t mean to assert anything. I’ve only pointed out a funny connection between the two quotes.