Preparing to installtion new Br111 5/16" solid harwood. I plan to stick new subfloor (to bring up level) overtop existing subfloor (house is only 15 years old - expect to find existing 5/8" or 3/4" OSB subfloor). Would it be wise to place a moisture barrier in between existing subfloor and new subfloor. Any harm here? I don't have moisture problems (dry crawl space in VA), but was thinking of just having added protection. I've read some posts that moisture barrier under solids hardwood over subfloor crawlspace is not recommended. Is this true? How bout sandwhiched in between 2 subfloors?

We have discussed these things many times here. Perhaps consider doing a search of this forum because there is a lot of great info here on your questions. But briefly:

Adding more plywood to an existing subfloor can be advantages when the new hardwood flooring is thin, when the subfloor is bouncy and flexes too much, or when one is going to glue the flooring down. There are other good reasons as well. You could place some 15 lb. asphalt saturated felt between the existing subfloor and the new plywood UNDERLAYMENT if you are not planning on using any adhesive and will be screwing it or stapling it securely. It's not a MOISTURE BARRIER. It is a MOISTURE VAPOR DIFFUSER. Plus, when putting a bunch of fasteners through it, it is less effective. The correct way to install a vapor barrier is to install 8 mil black polyethylene plastic under the entire house on the earth in the crawlspace.