Hello Jesper,
The master/slave system will make the playbacks on both units play the same playbacks and cues. For example if the master is playing cue 3 of playback 1 then the slave will do the same.
I do not know your setup. But if you use a playback per zone you can use to the playbacks without master/slave. This allows you control all zones independently for each other. If you want multiple zones to be "grouped" together you can use master/slave.
Best regards,
Guido