I would expect that lines three and four of "Old Walt" can refer to the work of both Whitman and Hughes because each man did the same thing when they wrote: each engaged in the activity of identifying questions and seeking answers.
Hughes' poetry is addressed to a specific point, his ideas some times more easily accessible in his verse than at other times. Hughes was concerned about speaking to hearts, and finding "answers" to questions some people might not even have known they had. Hughes could write simply and straightforwardly as he does in "Old Walt" or "Little Old Letter," or he could startle his readers with awe in poems like "Frederick Douglass: 1817-1895." In either case, he poetry cannot be ignored, even today.
When Hughes states that there were always more questions than answers found, I believe he saw this is Whitman's poetry and prose, and identified with it as an experience he was familiar with himself. I believe that Whitman was searching all the time looking to share the truths he discovered in observing life, and that Hughes did much the same thing in his poetry.