Here's my goal:
2 gameobjects which might be oriented any direction at start.
1st object is rotated (by outside force or script) and the 2nd object stays synchronized.
I'm new to quaternions

they feel a bit like sorcery…
I think that I store the 1st object's orientation, invert it, then multiply that against the object's current rotation (every frame).
that should give me a quaternion that is just the current rotation without the original orientation….
then I set the 2nd object's rotation (every frame), multiplied against its inverted original orientation.

Does that sound correct?