Just saw this post (I have been gone to the wilds of northern Manitoba for a week and just got back).
It sounds like what you have is a set that is the first two slices from the outside of a case hardened board. The cupped half is the first slice and the flat one, the second. The cupping is not related to a moisture difference, it is wood cell damage during too rapid drying.
What happens, and some woods are more prone to this than others, is that when boards/planks are dried too fast, the outside dries way faster than the inside of the board and tries to shrink. This also tries to compress the wood cells on the inside of the piece (which have not begun to shrink since they are still at a high moisture content). At a later stage of the drying these inner cells will shrink as they dry, but since the outer "skin" of the board has already taken a set from drying, the inner cells end up smaller than the outer cells. This sets up stresses in the board which shows in the cupping of the outer slice. Hope this is not too confusing
You can sometimes temporarily relieve this with heat, moisture, etc., and can often flatten the slice so that you can thin it (and keep it fairly flat because it is then more flexible), but you have not really eliminated the basic problem which is wood cell damage. There is no quick fix that really gives you stress free wood. Two things that are longer term solutions seem to work. First, if you suspect case hardening, you can rehydrate the board and then dry it again slowly. One thing that I almost always do when I suspect case hardening (for example when I get cocobolo that I know or suspect has been kiln dried - almost always has some degree of case hardening) is to store the boards for a couple of years under conditions of high seasonal moisture. The boards shrink and swell over a few seasons and pretty much even out. I let them slowly air dry another 6 months or so and then resaw, most of the problems are gone.
The other fix is one for after the case hardened wood has been resawn and you have the cupping. In this case, I soak both slices until they are back to green moisture content. This may take a week and the surface may get a real ugly color. I then sticker and dry slowly and things usually come out fine. You cannot just treat the cupped half because you need to restore both slices to the same condition.
Now, this does not solve your problem of wanting to get on with your guitar build, but hopefully gives some information that might be useful in the long run for folks.
Grant
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