Synchronizing Two Servo Motors

I'm building a gantry router with dual lead screws to drive the gantry, and
I need a way to make sure they never get far enough out of sync to rack the
gantry out of shape. I thought about mechanical linkages but I want to use
two motors for the extra speed and power, and I already have the motors.
Does anyone know of a circuit that can take two encoders as input, plus the
step and direction inputs, and output a fault signal if the two encoders get
out of step? There are several ways I could build such a gizmo, but figured
I'd check first to see if I can avoid reinventing the wheel.
--
Phil Mattison