Rat do die breeding for food does take it's toll on them so depending on their age and if they were about the same age this could be an explanation

You mention sneezing, there could be several issues, the most common dusty bedding which is not really a big deal and it does happen.

Bigger issue URI, you animal got sick the other became sick etc, hence why quarantining is very important when bringing new blood.

With proper ventilation, regular cleaning (so ammonia does not build up), healthy stock, proper quarentine etc URI should not happen or should be rare.

I have kept 1000's of breeders in the past 6/7 years and have kept them as low as 48 degrees and never had an issue with URI