In the dual setup I am having some issues with the Y axis belt and vibration when printing fast. the belt gets caught on the pulley as it vibrates and the grooves in the belt catch on each other and then pull the Y axis out whack. I tried a smaller 625 bearing and that cause some other issues. In the single configuration I pulled up on the belt after the pulley and it kept the vibrating belt away from the pulley. I can play with the tension of the belt but It goes from way more vibration on the belt to causing the motor to cease from too much tension.