Attached is a file, "Sketch31" cant be made into a sheet-metal part, but if the construction curve is deleted it is possible, which doesn't make any sense at all.
I guess I am confused with your request in order to offer any help. I just added a base flange to the part created with a mirror part feature. But because both parts are the same P/N, the flange appears on both parts.
I bit confused with what is your issue. There is only one part and assembly in the zip file. Can you attach some screen shots to show the issue?