In the active-voice example, the first sentence, "the men" are the subject. In the second, the men are the object. In the first, the usual sense would be that the men were doing the interviewing; in the second, that they were being interviewed.
However, there is a modern usage of the first where they would be the ones being interviewed, despite being the subject. This is because "to interview" can now mean "to be interviewed for a position, job, role etc". So "both men interviewed separately" can mean the two men applied for a job, both got to interview stage, and the interviews were conducted separately.