There is no evidence that giving the common pediatric vaccines simultaneously diminishes their safety or efficacy.
Studies show that providers are more resistant than parents to giving several shots at one visit.
Parental Compliance with Multiple Immunization Injections. KA Woodin, et. MI Horn and AM McCarthy. Children's Responses to Sequential Versus Simultaneous Immunization Injections. It is also preferable to avoid administering two intramuscular injections in the same limb, especially if DTP is one of the products administered.