Solid wood floor on floorbaords

Had to Google that one - never heard of the stuff. It seems to be used a lot by the Americans, although I'm struggling to find any UK sources for the stuff - is it available from any of the usual suspects? Not listed on B&Q, Wickes, Screwfix websites and cannot find a local shop via Google.

Some websites say it reduces squeaking, should the two layers of wood rub together, but if they're repeatedly nailed to one another over several strips of tightly locked T&G I'd imagine that's an unlikely scenario (surely the squeaks are more likely to come from vertical movement of the floorboards on the joists or passed each other on NON T&G flooring?) Some even suggest it can actually TRAP moisture!

It seems to be useful as an underlay over draughty floors with large gaps between the boards, but that isn't the case here.

I have to ask: why would there would be moisture trapped between the two layers of wood, the bottom layer on 7" joists well above ground level and with a heated and airy room below; and how does having a layer of moisture absorbing material trapped between the two layers help?