[Kevin Sevcik]

What ether said. The key question is if your pid loop is controlling position or speed.

If you're controlling speed then it's nearly hopeless, since any speed errors will build up over time into large position errors.

If you're controlling position, then it's relatively easy. I'm assuming you have basic familiarity with position PID loops, though. With that caveat, you have two options:
1. Pick one of the screws as a master. This is the only screw you directly command a position for. Set up the other screw as a slave by passing the master's feedback position as the position setpoint/command for the slave. pro: slightly easier to set up, guarantees the slave will be at the same position as the master, even if the master hangs up. Con: slave will always trail the master, since the best it can do is zero error to the master position, and there's bound to be some error. Best for really tightly controlled servos.
2. Two separate pid loops, one for each screw. Command for both pid loops is a single position value that you slowly ramp from one position to the next. Instant changes won't work, because each screw will try to get there as fast as possible, which means at different speeds. Slowly ramping the commanded position means each screw follows the ramped position second by second, which means the screws go up evenly since they're both trying to hit the same target. pros: even movement aside from whatever error is in your pid. Con's: somewhat more complicated, you need to ramp the position somewhat slower than the top speed of the slower motor. Ramping the position too fast means the motors can't keep up and it basically turns back into the instant setpoint change scenario