Questions about how the Broncos might specifically use rookie Montee Ball and Ronnie Hillman this season probably won't be answered until training camp wraps (and that's a best-case scenario), but I checked out a video on the Broncos' official team website that gets into the topic.
Most notably, analyst Andrew Mason recalls how John Fox's running backs have been used in the past: as an interchangeable tandem. One plays one series, the other takes the next series and if one gets hot during a game he sees more reps. If both backs can do the same things then there's no need to replace one during a drive, keeping the offense in a no-huddle rhythm. Plus in the event a running back were to get hurt, the other would be prepared for every down and distance.
Think about what happened last season when Willis McGahee went down. A lot of people thought Hillman would be "the guy" but it was Knowshon Moreno who stepped in. Why? Because he knew the pass protections better than Hillman and was an effective, experienced rusher too.
If the Broncos continue to groom Ball and Hillman and are comfortable with both of them as pass protectors, it's very likely they'll rotate based on possessions and not down/distance during the course of a game. If one or both can't handle all of that, Moreno will jump into a decent spot.
All of this will only cause headaches for Fantasy owners since platooning reps not only limits potential but it also makes the running backs very unpredictable. If this scenario plays out I'm not sure I would spend a top-70 pick on a Broncos rusher.