I just googles NRR and I was surprised about the addition of two
devices on each ear:
How does wearing dual hearing protectors change NRR?
When dual protectors are used, the combined NRR provides approximately 5 - 10 decibels more than the higher rated of the two devices. For example using disposable ear plugs (NRR 29dB) with ear muffs (NRR 27dB) would provide a Noise Reduction Rating of approximately 39 decibels