During an impetuous order spent by a synchronised peripheral and controller, can one of them choose to idle if the other moves? Or if the impetuous order is spent do all elements have to move.
If you declare "Move," both have to follow the rules for Impetuous Movement, as both are Impetuous. So yes, they both have to move, and they have to follow the rules for Impetuous when they do so. Note that if the Synchronized Peripheral is faster than the Controller, you run the risk of the Peripheral zooming outside of coherency, which can be less than ideal.