Please see my comment to the question. You don't need to do anything special to use one of the two 64-bit instruction-set architectures (x86-64 or Itanium, IE-64). To run it, you need that one of these architectures was on your hardware (CPU).
But further consideration is this:
there is no a way to call a function of one executable module from some other executable module
in the same process, if the two are compiled (targeted) to different
instruction-set architectures. The only resolution could be separate processes collaborating over IPC. Also, on Windows, x86 is also always supported for any of the 64-bit architectures under
WoW64.
That's all.
Please see:
http://en.wikipedia.org/wiki/Instruction_set_architecture[
^],
http://en.wikipedia.org/wiki/WOW64[
^],
http://en.wikipedia.org/wiki/X86[
^],
http://en.wikipedia.org/wiki/X86-32[
^],
http://en.wikipedia.org/wiki/X86-64[
^],
http://en.wikipedia.org/wiki/Itanium[
^].
Some advanced matter on instruction-set architectures and their interoperability:
http://en.wikipedia.org/wiki/X86_virtualization[
^],
http://en.wikipedia.org/wiki/X86_calling_conventions[
^],
http://en.wikipedia.org/wiki/X86_calling_conventions#x86-64_calling_conventions[
^].
—SA