Quarantine is not separate but close together. In what you described, they are sharing the same air, and therefore the same germs. To properly quantize, one must separate the birds by 300 feet, you need to care for one flock, change clothes, and care for the other flock.
Most back yard setups are not possible to quarantine. The thing is, if you can't do it properly, you may as well not do it at all.
Many people on here have closed, highly valuable flocks, or are emotionally attached to their birds that to loose a flock would be devastating. If that is your situation, do not add the birds.
Personally, I have added birds many times with no problem. Look at the birds, look at the set up, if they are sick, if they are dirty, if they look sick, do not take them, even if you feel sorry for them. But if they have bright eyes, walk well with good legs and feet, if they have good feathers, clear noses... well healthy looks healthy. They might be an excellent addition to your flock. Physically examine them for parasites.
Personally, I would never add chickens from an auction or the fair, they have been exposed to many strange disease, even if they went to the fair or auction healthy.
You have to assess your risk, and what you can live with. It is a real risk. People have lost their flocks by adding birds or not properly quarantining them. It is a real risk. However, many of us have never had a problem adding healthy birds.