Coronavirus:
https://www.vetconnect.com.au/5min/data/04820483.htm
Dr Dodds recommends the DHPP and Rabies boosters to be given separately, at least 4 weeks apart. This allows the dog's system to produce more antibodies (protection) against the disease being vaccinated against. When both are given at the same time, the little dog's system is then busy making antibodies against all the diseases, and the body can't make as many antibodies againt the each of the diseases as it can when separated by 4 weeks.
It's just a more effective way to vaccinate.
Also, dogs are more likely to experience a problem not on the first vaccine, but on the 2nd or 3rd. So just because a dog does well through one set of vaccines, there is no guarantee that he will not be adversely affected by subsequent vaccinations. I always stay in or near the hospital for an hour after my pups receive any vaccination, so they have immediate access to the vet and clinic for treatment, should it be necessary.
Each pup in any given litter will have a different amount of maternal antibodies present, depending on how well they feed initially. Pups may lose maternal protection anywhere from 6 -12 weeks of age. Any vaccine given when the puppy has active maternal antibodies will be killed outright, having no effect on the puppy's immunity. Pups really only need 2 sets of vaccines to become protected, as long as they are administered after the pup loses the maternal antibodies. So the later the vaccines are started, the better chance of them producing protection for the pup. They are given 3 sets, just in case the first set is rendered inactive by the maternal antibodies. To test each pup in the litter would be an expensive undertaking, so vets usually choose to vaccinate with 3 sets, thereby covering any lapses in protection, should a pup lose maternal antibodies early on.