Pikachu,
I was not sure what you meant in this post. Do you mean that "some vets only give half of the vaccinations needed for various diseases at one time" (giving parvo and distemper one visit, then rabies another day, for example) or did you mean that "some vets only give half of the vaccination dosage?
There is nothing wrong with dividing up the types of vaccines given on a single visit (although it does necessitate more visits) but you cannot give less than a recommended dosage of the vaccine required. This is part of the the reason that small animal vaccines come in singe dose vials.I know that it seems strange that a Great Dane and a Chihuahua receive the same amount of vaccine, but the immune stimulus requires a threshold "antigenic mass" in order to get a response. Giving half a dose of vaccine will probably result in lack of immunity.