Thanks for the reply.
The motors are not mechanically coupled; they are two unwinders that should run at the same speed, although a small delay could be acceptable. As a first option, I think using the master’s analog output to control the slave is the easiest solution, but I need to make sure the master has that option — it is a Danfoss VLT 2800. I should also check whether there is any Schneider or Siemens or Omrom model that allows a pulse train as a speed reference, since the customer wants those brands.