In pre-stressing work, if more than one wire or strand is included in the same duct, why should all wires/strands be stressed at the same time?

If wires supports are stressed individually inside the similar duct, then those stressed strand will allow against those unstressed ones and trap them.

If wires/strands are stressed individually inside the same duct, then those stressed strand/wires will bear against those unstressed ones and trap them.

Therefore, the friction of the trapped wires is high and is undesirable.

Agree with both answers given by Mr.Alex and Mr. Shakeel

Follow up...thank you Mr. Muhammad Shakeel

I am agreeing with answers given above and have nothing to add...

otherwise the stress in the wires can diver, and that rules out one of the wires! very dangerous

(Thanks . I agree with ( ALEX, KLAAS  & SHAKEEL 

Endorsing answers given by other experts