Aha ! Is the above multi-arch config what would result if I did :

  --with-universal-archs=ARCH
                          select architectures for universal build  
                         ("32-bit",
                          "64-bit", "3-way", "intel" or "all")

ie: --with-univeral-archs='64-bit 32-bit' 

??

The fact that the python --configure ignores my '--libdir=/usr/lib64'
setting and installs files under /usr/lib regardless suggest this
would NOT work as I expect.