Any CPU dependent on C++/CLI dependent on native C dll (any cpu for c++/cli)

What you describe is known as "side-by-side assembly" (two versions of the same assembly, one 32 and the other 64 bit)... I think you will find these helpful:

• Using Side-by-Side assemblies to load the x64 or x32 version of a DLL
• http://blogs.msdn.com/b/gauravseth/archive/2006/03/07/545104.aspx
• http://www.thescarms.com/dotnet/Assembly.aspx

EDIT - as per comment:

Here you can find a walkthrough for exactly your scenario: .NET DLL wrapping C++/CLI DLL referencing a native DLL

For me solution was following:

1. Build x64 bit version of C++ library to output file xxx.x64.dll

2. Build x86 bit (Win32 platform) version of C++ library to output file xxx.x86.dll

3. Add them as content files to my wrapper C# project. Ex.:

   <ItemGroup>
     <Content Include="..\..\$(Configuration)\xxx.x86.dll" Link="xxx.x86.dll">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </Content>
     <Content Include="..\..\$(Configuration)\xxx.x64.dll" Link="xxx.x64.dll">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </Content>
   </ItemGroup>
   

4. In C# Import functions from both x86 and x64 library versions. Ex.:

       [DllImport("xxx.x86.dll", EntryPoint = "FunctionName", SetLastError = true, CharSet = CharSet.Unicode, ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
       public static extern void FunctionNamex86();
       [DllImport("xxx.x64.dll", EntryPoint = "FunctionName", SetLastError = true, CharSet = CharSet.Unicode, ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
       public static extern void FunctionNamex64();
   

5. In C# implement function that calls correct overload based on current platform. Ex.:

       public static void FunctionName()
       {
           if (Environment.Is64BitProcess)
               FunctionNamex64();
           else
               FunctionNamex86();
       }
   

6. Now C# project can be built as "Any CPU" and used by other multiplatform projects.

7. To distribute it as NuGet package, I use following NuSpec configuration:

   <?xml version="1.0"?>
   <package>
       <metadata>
           <contentFiles>
               <files include="any/any/xxx.x64.dll" buildAction="None" copyToOutput="true" />
               <files include="any/any/xxx.x86.dll" buildAction="None" copyToOutput="true" />
           </contentFiles>
       </metadata>
       <files>
           <file src="Release/C#Wrapper.dll" target="lib" /> 
           <file src="Release/xxx.x64.dll" target="content" /> 
           <file src="Release/xxx.x86.dll" target="content" /> 
           <file src="Release/xxx.x64.dll" target="contentFiles/any/any" /> 
           <file src="Release/xxx.x86.dll" target="contentFiles/any/any" /> 
       </files>
   </package>
   

Answer is mostly based on: Using a 32bit or 64bit dll in C# DllImport