Noses are kind of funny and spacing is not one of that part of the body's main attributes. Some children have deviated septums (bone in the middle), small nose spaces, and lots of mucus blocking (bad nose blowers). Distributing the vaccine over both nostrils provides better possibility of absorption and reduces the likelyhood that some of it will run out.
Wayne Yankus, MD, FAAP
expert panelist: pediatrics