Sorry, I misunderstood your term "soft link". I thought you were describing a clip-type master link. Clip-types are to be avoided because they are rated by the manufacturers at only 70% of the strength of the other chain links, and consequently are the most likely cause of chain failures.
So, if I understand you correctly, a soft link is the chain master link that is assembled and riveted by hand as opposed to the machine peened-over links created at the factory on continuous lengths of chain.
Soft master links will have the same strength as the other machine-made links if peened over properly. As Felix points out they're not reusable.
Since you're only adding two links then you need a new chain including a new soft link. If you were adding four links, I suppose you could splice-in two links with two soft links. Perfectly peened, of course.