Bender and Chriss both initially being labelled as "Power Forwards" is not why neither of them has succeeded. Both of them looked like they could play multiple positions upon entering the league, with Chriss as a possible tweener forward initially to now being more of a PF/C. Bender was initially a big who could also play on the wing but since then has been projected as a SF, PF, or C. If both of them showed enough talent, they could be played together, somewhat easily, but that hasn't been the case. Neither has seemed to earn more playing time based on what they've shown on the court, during games.
Maybe they look better in practice going against each other? That's a possibility but it's clear during games that neither has the skill to warrant more than 20 minutes of playing time and they only get that much in hopes of developing them. So maybe playing against each in practice has hurt them a little but not because they're competing for playing time so much as they're practicing against someone who isn't quite at a starting NBA level. That's the only thing that may be hurting them but I doubt they always pair off in practice so I can't see that being much of an issue.