This is from-
http://www.mikeholt.com/mojonewsarchive/AFCI-HTML/HTML/AFCI_Update~20030320.htm
12-2-2 NM “Romex” was introduced to allow home runs for two AFCI circuits, as an AFCI can not have a shared neutral (due to ground fault protection). However, at least one manufacturer (Cutler Hammer) offers a 2-pole AFCI for home runs. Standard 12-2-1 NM “Romex” can be used. The 2-pole AFCIs are available with/without common trip for overcurrent and overload conditions; the trip is always common for an arcing or ground fault.
