It has been happening more & more recently but this sort of setup is not supported. To save you from yourself here I won't even go into which ones. Furthermore on ANY given day with ANY update the ability can be taken away as Serato again does NOT support two Natvie SDJ devices at the same time.

What IS supported is the connection of Official Accessories to control those 3rd & 4th decks. Using a pair of Pioneer CDJ's or the new Numark NDX-500's along with the DDJ-SX, SZ or other FOUR channel controller (pointless and impossible to control without FOUR channels) works just fine.