I have fitted a 4 function loco decoder (NCE d13) plus a 4 function decoder (TCS FL4) into a Bachmann 66.
I was then able to assign the same address to both decoders, then re-assign functions 4,5,6 and 7 to the function only decoder.
So in theory you should be able to do what you are asking, providing your DCC system supports 12+ functions.
Zimo do an 8 function OO decoder, you could then add a TCS FL4 which would give you another 4.
I think the ESU sound decoders support 4 functions.