I am building a C# DLL for inclusion in a Unity project. Due to some dependencies, this must be a 64-bit DLL. The last time I checked, DLLs for Unity had to be built with .NET Framework 3.5 (although this isn't explicitly stated in the DLL documentation). Are there any requirements for bitness?
If you use the 64-bit version of the Editor then you need a 64-bit plugin and vice versa. If you are using the 64-bit editor but are also targeting 32-bit platforms you will need two versions of the dll.
Here is a related discussion that may be helpful to make it more clear.
Here is another discussion, that explains how to import and configure both plugins side-by-side.