The NRRs are additive in theory. In practice, not quite. Real world use brings in variables of fit. Foam plugs used according to directions aren't so bad, but earmuffs can vary greatly in effectiveness. One indication of this is the manufacturers' publication of two different NRRs for the same muff based on over-the-head vs. behind-the-head placement of the headband. Then there are differences in the seal due to hair, ear size, the curvature of your skull, and how much subcutaneous padding you have. Over time seal quality can diminish as the materials age and lose flexibility. For rifle and shotgun use there is the loss of effectiveness when a large muff is displaced by the stock. A well fitted muff with a lower NRR may well be more effective than an ill fitted muff with a higher NRR.
Don't forget that a 3db difference halves or doubles the energy of sound and keep in mind that those NRRs are almost always for continuous tones, not impulse sounds.
Gee, I'd love to see your data!