This setting determines how the RBs share time on the field. The first number of the split is for the first position on the running back depth chart and the second number is for the second position. For example, if you choose the 80-20 split option, then the RB in the first slot will receive approximately 80% of the playing time and the RB in the second slot will receive the remaining 20%. If specialists are set, third down back and short yardage back, then those players are used in those situations and this split is only used in the other situations.
Finally, the split is for the top two spots on the depth chart and not the players. This means that if Player A in the first slot leaves the game with an injury and all players move up the depth chart, Player B, formerly in the second slot, will now receive 80% of the playing time and Player C now in the second slot will receive 20% of the playing time.