Conditional compilation of same source as a 32 and 64 bit application

Conditional compilation of same source as a 32 and 64 bit application

We have a 32-bit windows application which uses C++ and good old Windows
application model. Is it technically possible to have a single source and
compile it as 32 and 64 bit by say turning a compiler switch or using some
macros?
Currently I don't think the code will compile as a 64-bit application,
I'll have to fix the compilation errors, in general how should I proceed
so that it compiles as both 64&32 bit app. What should I keep in mind?
What will be the challenges? Any comments and tips will be appreciated.
Thanks