We set up an X-2 to try and simulate the problem you described when pulling two strings at the same time. We were able to consistently pull both strings without any slipping. However that will only work as long as both strings are side by side when wrapped around the gripper drum, and particularly when being placed between the gripper jaws. If the strings are twisted, or overlap, when placed in the gripping jaws they will slip. But when next to each other, we pulled at 40, 50, 60+ pounds and did not have any issues.
To get gripper jaws really clean use a cleaning stone (it’s a lot like a small knife sharpening stone) and rub it along the diamond coated plates. Then use the rubbing alcohol with a toothbrush, or old shoelace. Don’t be afraid to rub using a little elbow grease. Remember the textured surface is diamond dust, and diamonds are harder than just about anything you will be scrubbing with.