selecting interfaces for each universe

How can you select which interface is associated with which universe?
I am using an x-factor & a SAA. i need the ssa to be universe 1 but when i plug in the x-factor that one gets assigned to universe 1 & ssa to universe 2.
Reason is my external dmx board does not work via dmx in on x-factor but does work via dmx in on ssa unit.
I am using an x-factor & a SAA. i need the ssa to be universe 1 but when i plug in the x-factor that one gets assigned to universe 1 & ssa to universe 2.
Reason is my external dmx board does not work via dmx in on x-factor but does work via dmx in on ssa unit.