Running back by committee is a popular strategy in which teams use two running backs, giving each a fairly equal share of playing time. This approach is often referred to as a "committee" due to the fact that there is no clear-cut lead running back being guaranteed a vast majority of the snaps.
A committee can be a great way to catch opposing defenses off guard. One of the running backs is generally a "bruiser," who is a strong running back that can be relied upon to gain yards up the middle on early downs. The other running back is usually a "speedster," excelling at running towards the outside of the field and catching passes on third downs or other obvious passing situations. The contrast in styles between the two running backs forces the defense to make constant adjustments in order to account for both power and tremendous speed within the same offensive drive.
Running back by committees, while beneficial in real-life, negatively impact each running back's fantasy value. Since each back is not getting nearly as many touches as a running back that dominates nearly all of the playing time, the potential for lots of fantasy points is generally much lower on a weekly basis. Thus, a running back by committee tends to be a situation that fantasy owners avoid at all costs.