Hi, I'm not sure if I'm understanding it correctly but it sounds like you have 1 image that you would like to appeare twice in a separate PSD or you have 1 image that appears in 2 separate PSD's... to do this, go to File>Place Linked and bring in the external file. When you update the external file, your images should be linked.
Here's a video showing you what I mean:
https://youtu.be/POfaX3g1oY8
If I missunderstood you, please provide us with more information.
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