Rotating both hands downwards I found my answer some days ago but I forgot how to reproduce it again, my character is in T-pose, and in the final Idle pose both of his hands must be down near his legs. I tried rotating around individual origins, tried R > Y > Y, there was that one option in the menu that allowed me to do what I intend but can't remember which one it was, it worked in pose mode and there was no need for mirroring every time because it happened automatically. Please help.
I don't know if is this that you are looking for, but there is an option to move the mirrored bones if they have names like arm.L/arm.Left and arm.R/arm.Right.

Its named X-Axis mirror and you can find it in the Options Tab when pressing T while in the Edit Mode.