I can't say for sure, but the other day when I was on the phone with the CM booking a ressie, he suggested I do just that for a better time. He was going to give me a table for 2 and a table for 4 (we are a party of 5) and then told me that they would actually seat us all together once we arrive and they see we are one party. He said the restaurants do this all the time. I know I've read on other threads that some people were actually sat apart, but he was very certain that the restaurant would not do that. He told me that the CM's are allowed to book that way and that the only restaurant that they are told they cannot do that with is CRT. He was not one of those CM's that seemed to know nothing about WDW. He was very knowledgeable about many things compared to some of the CM's I've spoken to in the past.
Anyway, it's by no means a guarantee but at least I would certainly ask if you could wait a little longer to be able to be seated together. The worst they can say is no.