Yup, one screw per side has worked most of the time. On two of the pinch welds i've done I'd added an extra screw on one of the sides, far outboard, because of how the liner was bending. Seems as though every vehicle has its own personality

A few tips for the passenger side if you have rear HVAC:
1) Before taking the liner off, use a small drill bit to drill a hole through the wheel well liner and make a mark on the firewall about between 1 and 2 inches away from where the HVAC lines are. Once the liner is off, pre-drill the firewall.
2) Cut away as much from the plastic HVAC line guide as possible. It sticks out further than it needs to and you'll gain almost 1/2" more clearance.
3) Because of the HVAC lines you'll want to tighten the screw on this side by hand. If you go too far the liner will start bulging out where the HVAC lines are.