This.without PnJ kamui duo rapes. with it, madara nearly kills them
This right here.This.
Unless Madara has some, at this point unknown, way of avoiding Kamui/returning from Kamui dimension (Izanagi maybe?) then Kamui Duo stomps badly.
Seriously, if the two fight together, no one can land a hit on them (Obito's Kamui can be shared via touch), but they can 1-hit-KO anyone they are able to see (via Kakashi's Kamui).