Not turning up without the TTE being aware of no-show is different from cancellation. My understanding is that if you book two RAC seats under same PNR with common berth and later one person does not turn up but TTE does NOT know this, you will obviously get the whole berth in practice.
However if you cancel or TTE becomes aware, than the first-in-line WL ticket holder (if any) will be alloted the now-vacant half-berth so as to become RAC in substitution of the no-show / cancellation.