The OSHA Technical Manual allows employers to add 5 dB for the second hearing protector. An example: if wearing an earplug with an NRR of 25 together with an earmuff (also with an NRR of 25), expect a combined NRR of 30. Keep in mind, however, that this rule of thumb is a general estimate. To learn more about the ins and outs of dual hearing protection, and how to improve accuracy of your estimates, see this SoundBytes article.