Hey, we just got two bantam cochins and in unknown breed today. But one of the cochins seems to be coughing and then sneezing, and I really don't want to lose her!
Be sure and quarantine these chickens for 30 days from any others that you have in case they may have something contagious. Is there any nasal drainage or foam in the eyes? Respiratory diseases are very contagious, and some may make carriers of your flock. Have you contacted the breeder about their symptoms? Antibiotics such as Tylan or oxytetracycline can treat symptoms in some respiratory diseases, but you don't want to bring chickens into your flock that have diseases.
Infectious bronchitis, infectious coryza, mycoplasma gallisepticum (MG,) and ILT are the more common respiratory diseases. There is also aspergillosis from mold. Chickens can sneeze from dust, feed, and other irritants. Good luck with your new chickens, and remember to get them tested if you see any disease symptoms, since many can make carriers of the whole flock when combined. Here is a good link to read about the most common diseases including the ones above: http://edis.ifas.ufl.edu/ps044