same rules as for 32-bit ActiveX controls apply. As you might know, on Windows a process must be 32-bit or 64-bit. There is no way to use a 32-bit DLL or an ActiveX control that is 32-bit in a 64-bit process and vice versa.
That said, there is plenty of good information on MSDN and elsewhere on the Internet how to prepare your code to be successfully compiled and built for 32-bit and 64-bit from one source code.
Below a few resources to get you started (in random order):