On Mac OS, commonly, both 32-bit and 64-bit versions are installed automatically though you may have a choice for cutsom install.
On Windows, the installer gives you the option to install either or both versions. When in doubt you should install both 32-bit and 64-bit versions. 64-bit versions are needed only if you are running a 64-bit host application (DAW). If you are running a 32-bit application on a 64-bit OS then you would still use the 32-bit versions.