SASless ..... after reading your post about three times I am still not quite sure what you mean .... complicated stuff that .....
Anyway, it's really quite simple with multicrew IFR to an airport. One pilot stays on Instruments for the entire approach - flys it down to minima and shouts DECIDE at the appropriate time. If at this point pilot2 says GO AROUND, pilot1 remains on instruments and carries out the go around procedure. If however pilot2 tells him "visual I have control" then pilot1 hands over control and pilot2 does the landing.
At any stage of the approach pilot2 can take control if he aquires the necesarry visual reference.
It is entirely up to the PIC of the aircraft to asign these duties however most PIC's tend to let the other guy fly the approach and do the landing at the end, since this gives them a better overview of the situation and they can monitor the flying pilot. And really - flying the ILS is the easy bit! Landing the Aircraft in 500m Vis is the problem.....
Woolf