I think the OP is interested in your methodology. Are the pairs configured as a Master/Slave, or are each pair sharing a DMX address? Personally, I solve this problem through addressing.
Ah yes. I did it through addressing.
In the first instance all the 3NX fixtures were set to DMX Address 1. With everything set to address 1, you don't have to worry that the master is always first in line. With them all set to address one, they will all react the same, no matter what order they are in. (talking about identical fixtures)
In the second instance:
Both "A" fixtures were address 65
Both "B" fixtures were 71
Both "C" fixtures 81
Both "D" fixtures were 87
I set them both instead of trying to slave anything.