From everything I'm reading here, it's not a bug, it's just that the secondary coaches only have an effect if they are a very high level coach, and it's really only a full season effect, not a short term effect. Or maybe a secondary coach of lesser skill will help a bit if the main coach is very highly rated.
I think if a coach is starting over and has the money, it might be interesting to spend a ton on coaching to see what happens to young players, but I doubt the payoff would be as high as just having good players to begin with. You're talking $100k plus per season for coaches. At D-III or IV, where most of us labor, you can get several pretty good players for that annual cost.