Over Ricky's career, he has averaged right at 90 yards rushing per game. Over his past four years (2000-2003), Ricky has averaged right at 95 yards rushing per game. If he gets the rushes, he can get 1,000 yards in 12 games, easily. In 2000, he had 1,000 yards in 10 games.
The right balance will have to be found for it to happen though. Ricky has always been a back that gets better with more carries, and he's a career 4.0 yards per rush guy. Ronnie Brown's speed will probably allow him to make more long runs than Ricky, so you could see Ronnie averaging between 4.5 and 5.0 yards per pop.
I think a more likely scenario has Ricky getting around 240 carries for right at 1,000 yards with Ronnie getting around 200 carries for right around 900 yards while also catching 50 passes for another 500 yards.