I upvoted "too many cooks spoil the broth (or soup)" because I think it most closely matches the paradoxical effect of reduced efficiency from adding people to a task.
I also thought of the Bystander Effect, but as I understand it, that effect is usually brought up about others who just stand around and do nothing - rather than doing the same thing less efficiently. But I also see that it could be used to refer to reduced efficiency or to people who want to be helpful but end up being no help due to the effect of others around them.
"Passing the buck" or "diffusion of responsibility" don't sound fitting for this kind of concept, as far as I see it. Those terms are focused mainly on people who are deliberately avoiding certain work, as opposed to two people honestly trying to do a good job but suffering a drop in efficiency as a side-effect of being put on the same task.
Finally, there is the concept of the "mythical man month." This term is from a book on software development, so it is not useful to anyone who doesn't follow software or project management jargon. But it does capture the idea that adding people to a project tends to make that project even later. This concept, however, is about training and other overhead, not about one worker assuming another will do the same task, as is presented in the question's scenario.
Therefore, assigning more programmers to a project running behind
schedule will make it even later. This is because the time required
for the new programmers to learn about the project and the increased
communication overhead will consume an ever-increasing quantity of the
calendar time available