There's really no such thing as two equal employees. People are different. For example, back when I worked at Boeing, there were two cashiers in the cafeteria. One young, one old. The line for the young one was twice as long, because she rang up people twice as fast, and everyone knew that. Union rules, which presumed that time on the job determines productivity, required that the older worker got paid more, and when layoffs came, the young one got laid off.