so I can't break the link since I want a modification of part A to lead to the same modification on part B, the symmetric of A...
To create symmetrical parts, you don't need to do it "in the context of an assembly"
(this complicates external reference)
Symmetry can also be created from the part itself.
(the principle of "correct positioning of the origin and basic plans" helps)
1. Open the room
2. Choose a plane or face of symmetry
3. Make: Insert/Symmetrical Piece...
And so, in the assembly, you just have to insert the parts, and the position thanks to the basic plans, and if necessary add a "symmetry" constraint
so you have to change your need again
at the beginning you have a room A which has its own name
then you make a symmetry of part A to create part B which is named differently from room A
now you tell me that part A is different from part A of the first assembly since its symmetry is no longer A symmetrical but A with modification so it's no longer part A
It's not very clear your story
@+ ;-(