This is a real issue for me as well, for two of my projects. I've created FB7850694 describing the issue.
christianr
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I have had this issue more times than I care to count.
After several emails with people at Apple that didn't get me far (they acknowledged the issue likely was on their side), I tried creating a new sandbox Apple ID in App Store Connect and signed into iCloud on the device, and added the sandbox cards again, and this worked.
This suggested to me that the sandbox Apple IDs can somehow become corrupted or otherwise unusable for this purpose.
Also reported here https://developer.apple.com/forums/thread/737471
The linker error says "duplicate symbols".
Did you build the files successfully individually before? You almost certainly have defined the same symbols (function, global variable, etc.) more than once across the two files. Since they are now compiled in one executable instead of two, you need to have unique names.
Or compile the two separately, in one target each.