I used ETC dimmer doubling a long time ago (albeit on Ion) and seem to remember that the doubled dimmers were patched with two separate DMX addresses, and the combining was done in the dimmer rack. I didn't do any of the rack config, but if I remember correctly, you patch them as two separate fixtures (desk channels). The first channel would be the address of that dimmer, and the second was that address plus a constant offset number, like 200 or 300 or something.
Edit...found this at www.etcconnect.com/Support/Articles/Dimmer-Doubling-DD-Multiplexing.aspx :
"Note that Dimmer Doubling is configured differently with Sensor and Unison. With Sensor, the "B" side of the dimmer is always controlled by the "A" or "Normal" DMX512 level + 256 (i.e. if Dimmer 4A is controlled by DMX512 address 4, Dimmer 4B would be controlled by DMX512 address 260). With Unison, the user can select the DMX512 address for each "A" and "B" side. The default "B" address in Unison is the address that Sensor uses."
