Is it normal for a two year old to have sinus infections at least every two months.?

May not be normal. Infants and toddlers can get 6-12 viruses yearly;if in daycare, double to triple that. If your child's diagnosed with bacterial sinusitis that often it's reasonable to look for underlying causes. As dr. Ferguson said, food allergies & exposure to irritants like smoke are common causes;respiratory allergies are a big cause, too. Serious causes are rare. Ask your child's doctor to evaluate.
I see a few that do. Some kids seem to have specific patern illness that represents their size/vulnerabilities. Toddlers have frequent respiratory viruses & may have food allergies that also cause xs drainage & predispose kids to rhinosinusitis. After a few in toddlers I like to have food allergy testing. That is one area where diet change might decrease frequency. Avoiding smokers/perfumes/candles etc also helps.