What you are suggesting sounds like you want to do a semi-normal high-gravity batch along with a two-penny beer (second run on the same grain bed) and adding the two together and boiling it down to get your desired volume. And, in the process of waiting for the second to finish, starting the boil on the first run to speed up the concentration process. Do I have that right?
Starting off, I concur with tallie about running longer than normal boils. I've ran my boils over the norms (~ +50%) and had slightly darker beer as a result. Aside from the slightly darker color, it didn't seem to change the flavor any when doing a full volume boil. (See next for the exception...)
However, with starting the boil with a smaller and more concentrated wort, you could potentially get some caramelization flavors as a result. I have had this happen to me when I first started brewing with extracts and didn't have a large boil kettle to work with. (I boiled a 5gal batch of extract in a 2gal kettle... can you say Sugar Daddy Candy?). Aside from the off character flavor, it was obviously darker than it should have been too.
Not to say these are unavoidable consequences... just that they are potential results. I don't honestly see the caramelization flavors coming through nearly as strong as in my overly concentrated example since I don't think your first run will be nearly that high in sugar.
All that as it may be... Considering your question is about a method of raising the efficiency of extracting sugar from your grain... Have you given any thought into trying Fly Sparging?
I've not done it myself; but, it seems as thought you are already heading down a similar path anyway and this may get you into less trouble. Food for thought.