I don't really see it as competing against each other. I see it as diversification. In the long run it will equal out anyway.
Your miners could potentially be working on the exact same shares and by doing so, they are competing. The chances are small, but not that small over time. That just means that one pool gets the payout and the other doesn't and in such a case, you lose on the overlapping work. It is a small percentage to be sure, but it is real, if insignificant.