If a child receives a different brands of Hib vaccine at 2 and 4 months of age should a dose also be given at 6 months of age?

Yes. If different brands of Hib vaccine are given at 2 and 4 months of age then the child should receive a third primary dose of either vaccine at 6 months of age. A 2-dose primary schedule (that is, doses at age 2 and 4 months) is only appropriate when both doses are PedvaxHIB.