There has been more than one academic text which has referenced the relationship between Kirk and Spock. I do not recall this specific quote being repeated in any of those books, but I'd have to look to be sure. However, "two halves of the same whole" definitely originated in the (non-academic) book Star Trek Lives! (Lichtenberg, Marshak, Winston, 1975).
Lichtenberg and Marshak: When we asked how Spock and Kirk regard one another, what in fact their relationship consists of in texture, Gene Roddenberry said, " . . . I definitely designed it as a love relationship. And I hope that for men . . . who have been afraid of such relationships . . . that they [Spock and Kirk] would encourage them to be able to feel love and affection, true affection . . . love, friendship, and deep respect. That was the relationship I tried to draw. I think I also tried to draw a feeling of belief that very few of us are complete unto ourselves. It's quite a lovely thing . . . where two halves make a whole."
Also, in another non-academic book, Shatner: Where No Man . . . (by Marshak, Culbreath, and William Shatner, 1979) there is this:
Marshak and Culbreath: "I know you've told us you designed that relationship as 'two halves which come together to make a whole."
Gene: "Oh, yes. As I've said, I definitely designed it as a love relationship. I think that's what we're all about — love, the effort to reach out to each other. I think that's a lovely thing. Also, dramatically, I designed Kirk and Spock to complete each other, and in fact the Kirk, Spock, McCoy triad to be the dramatic embodiment of the parts of one person: logic, emotion, and the balance between them. You cannot have an internal monologue on screen, so that is a way of personifying it, getting it out where it can be seen — that internal debate which we all have within And I designed Kirk and Spock, as I told you, as dream images of myself, the two halves. But in terms of the characters, yes. That closeness. Absolutely."
So Roddenberry was not looking at this as a romantic/sexual relationship, but along the lines of Epic Friendship. (And I say that as a K/S fan!)
In fact, in later years, when K/S reached underground fanzine publication, He remained . . . hmm . . . diplomatic? . . . neutral? . . . when communicating with fans on the subject. But it was not how he himself viewed these characters that he created, as witness this recounting in Star Trek Creator: The Authorized Biography of Gene Roddenberry (David Alexander, 1994), regarding Star Trek pro novels:
"In the mid-1980s, Gene's attitude toward the books changed when Pocket books . . . published a novel containing elements of K/S . . . . Gene investigated and he was livid. Printed Star Trek was taking a much different course than the one originally plotted by its creator. Gene was not a homophobe — anyone who knew him knew that, but these were *his* characters *He* created what they were and they were not going to be changed *that* much without his permission. . . . and the offending edition was withdrawn and rewritten with 30 pages excised."