You'd have to test it to know what the new absorbtion would be.
If material #1 had an NRC of 0.6, and Material #2 had an NRC of 0.8, then putting them on top of each other wouldn't give NRC of 1.4. They can't be added that way. As a limiting factor (not really accurate but as a rule of thumb), think of NRC as a % of energy absorbed -- there's no way to absorb more than 100% of the energy, and absorbing more than 99% of the energy is unlikely.
NRC is a weighted average of absorbtion of sound energy over many frequencies.
The addition of a second porous material (mineral wools) will increase the energy that's absorbed, moreso at the lower frequencies than the higher.
Some guess follow:
a) if it's more of the same material it will behave as the same material twice as thick
b) if it's porous over solid (3/4" plywood or drywall), then the solid can be ignored.
c) if it's solid/membrane over porous, then it's a membrane absorber.
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