Create your base sheetmetal part with the sheetmetal tools in the sheetmetal environment, save it and then:
Make new part (not sheetmetal)
menu: feature > boolean > unite and select the part you want to mirror click to place the part.
Menu: edit > commit to accept then feature > scale or mirror.
Now you have a linked mirrored version of the original part.
Make new part (not sheetmetal)
menu: feature > boolean > unite and select the part you want to mirror click to place the part.
Menu: edit > commit to accept then feature > scale or mirror.
Now you have a linked mirrored version of the original part.