In Computer Organisation Architecture, Why some devices perform dual operations and some perform single operation?
please answer if anybody know!

Not clear what you mean. Give an example?

Also, we don't use hashtags here.

Is that homework?

I presume that you have seen a fancy chart of some kind with the Roles neatly spread once or have learnt that these should be separated neatly.

The truth is: nobody got time/money/hardware for that. So a real net or diagram of it will never neatly assignee all roles to individual computers and will just plop some redundancy where the load could be too high/failure would be significant.

I hope that clears your confusion.
If not just ask.