There is a halachic idea called shome'a ke'one, that says that (in certain circumstances) one who listens silently to another who says a blessing can fulfil their obligation thereby. Different authorities have different parameters for in what context this is appropriate or what particular intentions/actions need to present in order for this to be effective, but the basic idea exists across the board.
In the cases I've seen discussed (and as is explicit here), the listener must hear the entire blessing out loud from the second person in order to take advantage of shome'a ke'one.
Is there any discussion of whether a partial use of shome'a ke'one works? That is, if the speaker doesn't say the whole blessing out loud, but the listener says themselves the unheard words, is the listener's obligation fulfilled?