first_build.py

# Imagine this is first/first_build.py

from cffi import FFI
ffi = FFI()
with open("/usr/local/include/tfc.h") as f:
  ffi.cdef(f.read())

ffi.set_source("first._first",
               """
               #include "tfc.h"
               #include "first.h"
               """
               include_dirs=["/usr/local/include", "projects/first/include"],
               library_dirs=["/usr/local/lib", "projects/first/lib"],
               libraries=["c", "tfc", "first"]
               )

if __name__ == "__main__":
    ffi.compile()

second_build.py

# Imagine this is second/second_build.py

from cffi import FFI
ffi = FFI()
with open("/usr/local/include/tfc.h") as f:
  ffi.cdef(f.read())

ffi.set_source("second._second",
               """
               #include "tfc.h"
               #include "second.h"
               """
               include_dirs=["/usr/local/include", "projects/second/include"],
               library_dirs=["/usr/local/lib", "projects/second/lib"],
               libraries=["c", "tfc", "second"]
               )

if __name__ == "__main__":
    ffi.compile()

setup1.py

# Imagine this is first/setup.py

setup(
  name="first",
  cffi_modules=["first_build.py:ffi"]
  )

setup2.py

# Imagine this is second/setup.py
setup(
  name="second",
  cffi_modules=["second_build.py:ffi"]
  )