Rat Quarantine and other issues...?

I had two hooded rats up until January 5th when one passed away of old age. :(

After a few weeks I bought another rat. He is albino and I was told he is about 4 months old. Tomorrow will be two weeks since I brought him home. I have had him in quarantine but yesterday, I read that it is pointless unless their air supply is completely separate... which it has not been. The new rat started sneezing/coughing (not sure which although he has been pawing at his nose) in the car on the way home the night I got him. I thought that it was because the air in the car and in my apartment was dry so I started running a humidifier. He is still coughing/sneezing but seems perky and curious about his new surroundings. Bottom line, it is pointless to continue to keep them separated seeing as how they have been sharing the same air for two weeks? Is there anything that could be causing the sneezing/coughing that is NOT contagious? Any other advice would be great too!