Blender Version
Broken: 2.90.1, 3e85bb34d0d7, master, 2020-09-23 06:43

Short description of error
When I try to copy the pose of bones in different armatures, only one of the armatures is copied.

Exact steps for others to reproduce the error

1. 1. Create a two separate armatures with bone.
2. 2. Select both and go to pose mode.
3. 3. Do some manipulate with both, and insert keyframe for your changes.

4A. Select any bones in both armatures in one time. Press Ctrl-C, select other frame, press Ctrl-V. And insert keyframe for comparing.
4B. Select keyframe in Action Editor, duplicate or copy to other empty frame.

5. Compare keyframes.