The OP means that Gardner often had co-stars that she worked with, but never shared scenes with them in other movies. She does not share any scenes with Burt Lancaster in SEVEN DAYS IN MAY or THE CASSANDRA CROSSING. Gardner worked with Anthony Perkins in ON THE BEACH, but does not share a scene with him in THE LIFE AND TIMES OF JUDGE ROY BEAN for instance.

She had onscreen time with both Lancaster and O'Brien in "The Killers".
"No, I don't like to cook, but I have a chicken in the icebox, and you're eating it."

She had plenty of scenes with Heston, including the final plunge.
Her smaller roles in "Seven Days in May" and "The Cassandra Crossing" did not require those characters to have scenes together. (In "Crossing" she was one of the passengers on the ill-fated train. Lancaster was miles away, doing his evils deeds. The characters were not even aware of one another. Sophia Loren didn't share any scenes with him either, and she was the star of the movie.)

Very, I agree, especially if they were both marketed as co-leads.