You can instantiate a 64-bit com server from a 32-bit client if the server is an out-of-process server. A COM server hosted in a 64-bit DLL can be instantiated as an out-of-process server by using the system COM surrogate.
Example usage update -
The screenshot below shows a COM server hosted in a 64-bit DLL being instantiated by a 64-bit client directly and as an out-of-process server using the system's COM Surrogate.
It also shows a 32-bit client failing when attempting to instantiate it as an in-process server but succeeding when using the COM surrogate to instantiate the 64-bit server as out-of-process.