Why do some gas masks have two filters?

Why do some gas masks/respirators have two filters?
All about dual filter gas masks, the purpose of dual filters is to make breathing easier, as two filters offers less air resistance than one filter as well as doubling the life of each filter.
However the downside is that both filters will need replacing at once and having two filters adds additional weight to the mask as well as making it harder to shoulder a rifle.

Masks in the video are:
Scott GSR/General Service Respirator
Soviet PBF cheek filter mask
Avon CT12

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