In general sense of "use", in principle, you can use them, but never directly, no loaded in one process as you usually do.
If people have two pieces of code without the source, targeted to different
instruction-set architectures, they wrap one or more of them in some service and organize communication through IPC. In other words, as no executables (EXE, DLL, anything) can work in the same process, you should have more than one separate process, and, as processes are isolated, they need to communicate via IPC. Please see:
http://en.wikipedia.org/wiki/Inter-process_communication[
^],
http://en.wikipedia.org/wiki/Instruction_set[
^].
Also, I'm afraid you are missing one important point: there are at least two different 64-bit instruction-set architectures: x86-64 and Itanium (IE64); they are incompatible but both can run 32-bit x86 applications:
http://en.wikipedia.org/wiki/X86-64[
^],
http://en.wikipedia.org/wiki/Itanium[
^].
—SA
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