If you want constant contact, I think you'd have to set the spring force to be such that the force applied by the belt to the pinion creates a torque on the each pinion that is equal to or exceeds the torque you apply to turn the drive pinion (the one connected to the motor) via the timing belt coming from the motor. You could either figure this torque as being the max that could be generated by your motors or the max you think you'll encounter given your acceleration settings and anticipated max cutting load.
CNC mill build thread: http://www.cnczone.com/forums/vertical_mill_lathe_project_log/110305-gantry_mill.html