In On Beauty by Zadie Smith, do the phrases "only connect" and "do concentrate" apply and reflect the text? If not, what pair of phrases does?

This is an interesting question. I can see how "only connect" and "do concentrate" might be a representative pair of phrases to give a metaphysical and symbolic representation of Zadie Smith's On Beauty.

"Only connect" may be symbolically representative of the Belsey's and Kipps' need to and inability to connect to the right expressions of identity (except perhaps Kiki). This may be seen as symbolically illustrated when Howard bends down to connect the cord of his old projector to a power source even while Miller stands in the doorway explaining that "pah-point" [PowerPoint] is the newest best technology, saying "Ah could show you."

[Howard] got on his knees and plugged the projector's cord into the wall; [but] a snag of blue light lept from the [wall] socket ... He twiddles the connected cable. He pressed hard on the light box hoping to engage some loose connection.

"Do concentrate" may be representative of the need for various characters to concentrate on elements of living that seem to get buried under other indirectly related elements of other people's lives. For instance, Howard's scholarship on Rembrandt--on which he can no longer concentrate--is buried under Kipps' newly published and acclaimed work on Rembrandt. Another example is that Kiki was unable to concentrate on her potential friendship with Mrs. Kipps because the friendship was metaphorically and literally buried in Howard's confession of unfaithfulness and in "the magazine rack" and again in "her Alice Walker Barnes and Noble tote bag."

Another representative option might be the pair of phrases that end the novel and lead to the option of reconcilliation and new beginnings that Kiki and Howard experience:

Everyone was enjoying the sense of new beginning and fresh pastures that ...cycles offer ... They were starting again.

These phrases are "on beauty" and "being wrong." The world comes to right itself to some extent--after wrongs committed--in one of its cycles of movement forward, and beauty is redefined and rediscovered in perhaps unlikely ways and places. Kiki and Howard make just such rediscoveries the night of his big presentation with his projector:

The surface of the water was dark, reflective--a cautious bather could not be certain of what lurked beneath. Howard looked at Kiki. In her face, his life. Kiki looked up suddenly at Howard--not, he thought, unkindly.