Chaus I'd suggest going and reading DMX too, First off any device that uses DMX to control on each universe only goes from 0-512, To change universes you have to connect the cable that is connected to the second universe of the console to the device you want on the next universe. There is no setting on any fixtures that i know of to differentiate different universes on the same cable. (nor could I fore-see using two universes on one cable).
Correct me if I'm wrong, and maybe I'm just nit picking or misunderstood what you meant, but with a Robe Robin MMX Spot (user manual) you could set the universe on the fixture while using direct Ethernet control, because Cat5 cables are able to control multiple universes using Artnet or a related protocol.