You should not combine vaccines in the same syringe/injection site that are not labeled to be used together. Differant vaccines have differant adjuvants and antigens that can interfer with each other and cause a poor immune response. When using multiple vaccines it is best to put each injection in a differant location, neck, hip, rump, to allow the body to raise the best immune response.
This is probably nothing the same, but I take two different kinds of insulin. A fast acting one before meals and a long acting one at night. In the product literature, it stresses not to mix or even use the same syringe as if you do and they mix at all, the long acting one is ineffective. Learning this only confirmed to me not to mix them (but I do use the commerically combined shots, like WNV+EWT), 5-ways for pregnant mares, etc.