What is the mower? I'm presuming it is hydro drive, right?
My Exmark has an adjusting knob that enables me to make fine adjustments, one wheel against the other. A turn on the knob will change the position of the hydro pump controls -- makes one wheel turn faster, the other slower. By fine-tuning, both wheels will be going exactly the same speed -- mower travels straight ahead.
Pistol grips should have nothing to do with the matter. The issue is getting the controls on the hydro pumps perfectly matched.