Using two coats will probably increase DMAX of most papers, although some don't take too well to the second coat, and have problems with blotchiness.

I have speculated that there are several things going on here:

1. There is more sensitized material in the paper, which will result in a higher DMAX. By doing two coats, you are able to get much more material into the paper fibers than would be possible in a single coat, because some of the materials are at saturation in the mix.

2. The first coat is somewhat acidic, which helps prep the paper for the second coat.

3. The first coat is wet, which helps make the paper locally more humid, which helps with the second coating. I never blow dry after the first coat for this reason.

4. The first coat starts to work on the sizing in the paper, which makes it more absorbant to the second coat. For this reason, I think that the first coat is important, but not necessarily solely because of the metal salts in the coating. That;s why a pure H2O presoak may be beneficial in the same manner, but I suspect it will not get the DMAX improvement that a two metal salt coat print will achieve.

Typically, high humidity is not as big of a problem as low humidity to my understanding. I have printed up to about 70-75% and it seems to work equally well, although there is more of a printing out image, which will affect both the speed and the contrast. I try to keep my RH between 40 and 60%, and this is not too difficult except in the heat of the summer.

I don't have any real experience with high humidity levels, so I can't really recommend a specific paper to overcome any printing problems you may have.

I think the biggest problem you may have is getting the paper to dry in a reasonable amount of time. At that high humidity level, you would probably have to help the paper dry to avoid fogging and real long wait times. I think it would be important that you don't dry it too much after the first coat, however.