The C406-2 series ELTs have 2 antenna coax connections. One for a 121/243 ELT antenna and one for a 406 ELT antenna. The C406-2 is a dual output ELT as was the case with all original Artex 406 MHz models. Therefore, more or less, the C406-2 can be considered an earlier design of the 406 MHz ELT from Artex.
Since Artex has the C406 series of ELTs which incorporates one single coax for all 3 ELT frequencies, there would not be any practical reasons to install the C406-2 unless you do not want to uninstall the original 121/243 ELT antenna from the aircraft or you are following some sort of aircraft OEM service bulletin. Some original Artex ELTs may make it easier to install the C406-2 as a replacement as well although this is rare.