Are you setting the original morph back to 0 before you try usning the mirrored morph?

I'm guessing that the necklace is a single OBJ? That why the mirroring is moving the whole necklace.

You can try MASA's MTMirror, goto Download & Other tools on the side menu at the MASA Poser++. Though it would require a bit more work, with you needing to have both the base and morphed item as OBJ.

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Consider me insane if you wish, but is your reality any better?