other than mentioning having to use an inproc server...
This is not possible in-process, a 64-bit process cannot load and execute any 32-bit code. This would only be an issue if you have a dependency on unmanaged code, pure .NET code can always be jitted to the required architecture. A very nice goody provided by just-in-time compilation.
The only way to make such unmanaged code work is out-of-process by using a 32-bit helper process to give the code a compatible home. You get this almost for free if the unmanaged code is provided as a COM server, you can configure its AppId to run the server in a surrogate process. Just a matter of setting the registry keys correctly. If not then create your own .NET host application with the Platform target set to x86 so it will run as a 32-bit process. Get them talking to each other with a .NET process interop mechanism like WCF, Remoting, named pipes or sockets.