I want to add a 2 gang outlet to an exsisting box with 3 white & 3 black wires . Not sure of the best way to do it. Don't know if I can split the wires and tap into each one seperatle or add wires to both the black & white triples. Any help would be appreciated.

What do you mean 2 gang outlet ?what another recp? Into a single gang box?what was in there to begin with?