I have added a second vertical thruster to USR1 (Under Sea Rover 1). This eliminates torque twist and increases the vertical thrust available. Next step is to add pitch angle control capability. You will see some twitching in pitch due to Kp being too large in pitch rate (gyro) control.
Instead of adding a second vertical thruster, unless you really needed additional vertical thrust, would have been to offset the torque twist by increasing the thrust of the appropriate horizontal motor depending on if you were going up or down vertically.
I needed the additional vertical thrust. In the relatively high current environment where I work, I need a lot of negative buoyancy to get down in a hurry. To offset that I need more vertical thrust. Of note though, I have yaw dampening which initially corrects the torque twist, but then it starts drifting. The extra vertical thrust solves two problems for me.