Both libraries have files named exception.cpp, stdexcept.cpp, typeinfo.cpp.
If you want to build both libraries together, libstdc++ style, and all objects are in the same build directory this will cause name clashes.
While it’s a trivial problem, it is a source of possible confusion and problems, and it’s probably better (and very easy) to rename libcxxapi source files to different filenames than libcxx.
Yaron