Hi Thielert,
I believe that to do this safely, will require a slave axis and a master axis. Servo motors rely on encoder feedback to work, so each motor will have to have its own encoder. For proper position monitoring, each motor must be driven as an independant axis, and the controller takes care of the master-slave aspect so that they move in unison. This is more or less just an opinion I have to offer. There might be some sneaky way around this.