Stress on its own won’t kill, it will make the fish more susceptible to pathogens.To me those are the two most logical reasons of death of the two fish.
Really, ask the LFS their salinity and let us know. I try to limit salinity rise to 0.003 per day max. Anything more than that and some fish may not physiologically tolerate it. For example if your LFS runs 1.018 and you put them into 1.026… they can die of dehydration.
Otherwise I agree with above… ammonia or velvet or brook would be first guesses.