This might sound like a stupid question, but...

...Should I go for 4" thick Rockwool or will two pieces of 2" Rockwool give the same absorption characteristics? I can get 2" thick stuff in 2-3 days but the 4" stuff will take two extra weeks. Couldn't find it in a search on here. Thanks.

Any combination of thicknesses will work virtually identical, it's the overall thickness that makes the most difference. Exceptions would be if each piece has a membrane such as FRK style 703 - then, it would make a difference whether you used 2 2" or 1 4" layer... Steve

Thanks for the reply Steve. Tom.