Using a B.747 as an example ; Both outflows operate independently of each other and the chances of both of them failing completely are almost zero . Don't forget , they can also be toggled manually via both AC & DC motors As BAeng. says , in the crz they are just 'pulsing' and are almost closed anyway.
Should they both fail in the closed position , the emergency outflow valve will ping open automatically when diff-px reaches 9.25psi ..[I think it's 9.25 anyway]]
Should the aircraft make a somewhat hasty emerg. descent [i.e. outside px exceeds that of the cabin ] then the inward relief valve will automatically sense that and open up .
@ OP : What type of aircraft do you refer to specifically ?
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