So this basic assumption must be met. If they are synergistic then deleterious mutations do not pile up and become such a problem. That is the average fitness does not drop to as low as 0.05.
However, I know realize that I don't know how "average fitness" is determined or what it means. It seems to suggest that if it is 0.5 then half of offspring survive to reproduce. That makes some kind of sense.
Now then how do we decide if it is actually synergistic?
It makes sense to me that it might be. Obviously, there will be cases where having one moderately deleterious mutation might not be enough to stop reproduction but a second one is fatal? Correct?