Long before you begin playing fantasy football, you will have to set up your league. One of the most important settings is the number of starting running backs. Unless you set this properly, you may end up with managers that are apathetic and don't care anymore in week 7.
I have always argued for only 1 starting running back, especially in leagues that have 8 or more teams. I really feel that if you have 2 starting RB positions, your league will really struggle. When you have a large league, like 8-12 teams, having 2 starting RB positions, is too many positions. Besides no NFL teams actually start 2 RBs.
In the NFL today, fewer and fewer teams are starting and relying on a single running back. This means that the RB position may not be as important as it once was. Further, forcing managers to start 2 RBs just dilutes the talent pool further and may provide for some lopsided matches. The first 5 or 6 running backs that are drafted will probably be stars and score a disproportionate amount of points, while the rest will be average.
The key to success will be in drafting strategy (an entirely separate topic). If you want your league to truly be among the elite leagues, then you need to allow space for management. In order to allow for managing, rather than guessing or luck, you need to decrease the RB situation to only 1 starting back.
In an elite league, you should have a setup like below:
In doing a quick analysis, and going through about 5 or 6 mock drafts based on the 2003-2004 final season statistics; having 2 starting RBs will be a problem for larger leagues. There will be no management involved. Basically, managers will end up just starting everyone. Starting everyone is not very skillfull and takes away from the excitement.
Last year, I played in a 10 man league, where you had to start 3 QBs and 3RBs. That is insane. Many of the teams had starters that weren't even playing or were injured. Throughout the season, most managers ended up starting a 3rd QB that doesn't even play. That, is not very skillfull. When you are forced to choose between a probable Clinton Portis or a lackluster Edgerrin James; that is where the skill and mind comes into play.
Of course, there are large leagues that do start 2 RBs, and in the past it worked out. But in today's NFL, with fewer featured backs, starting "all you got" at one position takes away from the strategy and skill of the game.