Note that many operating systems today are already multiple architecture. It may not seem like it, but for example x86 and AMD64 are different architectures, j

Note that many operating systems today are already multiple architecture. It may not seem like it, but for example x86 and AMD64 are different architectures, just as 32 bit ARM and 64 bit ARM are different. The operating system has to define system call interfaces for them, have copies of libraries for each architecture, ensure the processor(s) are in appropriate modes when running the applications, provide inter-process interoperability even when different architectures etc.