With DD I was all set to split up all the big combo shots. My pedi was ok with it and I felt I had done my research. Then one of the other docs in our pedi practice explained something to me that I really understood and it changed my mind.
The big combo shots are updated frequently and modified frequently. In order to get the split up shots you will be accepting a vaccine that hasn't been updated in a very long time. It's even hard to get them now because they are no longer made by the big producer. I wasn't cool with giving meds that were designed about a decade ago vs ones tweaked and updated as recently sometimes as a year ago. I prefer my science up to date!
We ended up giving all vacs on time (except the Hep B which we started at 1 month instead of birth). I don't like my kids hanging around kids I KNOW aren't vaccinated until they've had all their up front shots at least (2 years old).