Create a tiny executable (create a batch file per the following pseudocode and convert to .exe using tools like battoexe, etc)
IF %PROCESSOR_ARCHITECTURE% == "AMD64" THEN
Install 64-bit version
install 32-bit version
Attach both MSI files as additional files and use the .exe created above as your source package
This will work but you will not get accurate reports of successful or failed updates since the script should always have a return code of 0. Managing multiple architecture packages really isn't a large hassle and having accurate reporting data is far more valuable than only having to approve one package versus two.