This happens to me too, and seems to have been reported a few times here and elsewhere (e.g. StackOverflow and individual blog posts), but Apple seems yet to address, let alone fix, the issue.

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.

Also reported here https://developer.apple.com/forums/thread/737471