I did some testing over the weekend regarding this question. My results are nothing but preliminary, but I thought you might get some use from the information.
I tested cutting the first coat with 50% H2O vs. a straight full strength coat. The second coat was full strength in both cases.
First, the paper seemed to absorb the 50% solution faster, so I ended up putting in a little more than 50% water, so that there was enough solution to enable consistant coating.
It appeared that I got comparable dmax values with both, although I can't test that right now, as my densitometer needs a new PS. However, they looked very similar in dmax.
I noticed that the 50% first coat seemed to result in a faster printing solution. I would guess that it printed 2/3 or 3/4 of a stop faster than the full strength solution.
The 50% coat also appeared to have a slightly smoother tone in the middle zones where it is most apparent. This could have been a coating varialbe, or it could be a product of the solution.
At this point, I don't see any drawbaaks to cutting the first coat with 50% water, and I'm inclined to think it may be a good thing to do, especially when printing larger, as it does result in a good bit of metals savings.
More tests are needed to determine if this is definately a win-win situation, but it appears to work well at first glimpse. Another test that might be worthwhile is to use a 1% oxalic acid solution instead of H2O for the first coat, and see if that improves that coating.
It may be possible to cut the solution strength of the first coat even further and still have the benefits of double coating, but I don't know how far that can go.