All 3 Uses of

inseparable
in
Atlas Shrugged

They came from very different backgrounds, but they were inseparable friends.†
Chpt 1.7 *

a face as alive as yours-a railroad platform, not a curtained apartment-you looked like a symbol of luxury and you belonged in the place that was its source-you seemed to bring wealth, grace, extravagance and the enjoyment of life back to their rightful owners, to the men who created railroads and factories-you had a look of energy and of its reward, together, a look of competence and luxury combined-and I was the first man who had ever stated in what manner these two were inseparable-and I thought that if our age gave form to its proper gods and erected a statue to the meaning of an American railroad, yours would be that statue......Then I saw what you were doing-and I knew who you were.†
Chpt 3.2

She threw her cape back, she stood defiantly straight, as he had seen her stand on the steps of the tower-as he had seen her for the first time, ten years ago, here, under the ground-she was hearing the words of his confession, not as words, but by means of that beating which made it so difficult to breathe: You looked like a symbol of luxury and you belonged in the place that was its source ....you seemed to bring the enjoyment of life back to its rightful owners ....you had a look of energy and of its reward, together ...and I was the first man who had ever stated in what manner these two were inseparable......The next span of moments was like flashes of light in stretches of blinded uncon†
Chpt 3.5

Definition:

not capable of being separated

or: people who and are almost always seen together -- usually close friends who like each other's company