This is a bug I've had repeatedly with coaxial helicopters and the like. Sometimes one rotor will just produce more torque than the other. It's not an issue with my craft, it doesn't matter if I exactly duplicate and mirror one rotor to make the other, the issue persists. Sometimes deleting and re-mirroring the part fixes it, sometimes nothing will. It's very frustrating and I've had to throw away a lot of perfectly good builds as a result.