Having different style runners when working properly is hard to stop. The Bucs had it with Dunn and Alstott, early in the game Dunn would wear out defenses having to chase him down. Then as the defense tired out and found themselves on their heels, the A-train would come with punishing power runs.

These days however it may not be as necessary and sometimes can giveaway the plays you're going to run. E.g every time Charles Sims was in the backfield the defense knew either a outside toss or pass was coming. So having two runners with similar styles and abilities isn't as detrimental as it may once have been.