In some cases, the dual-core MCUs are not the same architecture variant, which could complicate things if Mbed-OS was built to run on multiple processors at once.
I don’t know of an embedded RTOS that supports multi-core with one copy of the program. The intended use of a separate core in many chips today is as a separate network processor (such as in the nRF5340). Many times, dual-core MCUs have boatloads of flash (and some even support XIP from external flash) so I don’t see this as a huge issue. The build/upload process might be a bit more painful, but there are ways to automate that.
The way I see this working in the near term is:
Introduce some new configuration options that are similar to the Managed Bootloader ones already available. This would allow you to specify a pre-built “dual-core-binary” that can then be merged with the current build artifacts in a post-build step. You would also have to specify the “dual-core-start-address” and possibly “dual-core-memory-size” so the tools could merge the programs into one hex file to be programmed.
Embedded Planet has provided a port of NXP’s Embedded Remote Procedure Call (erpc) library for Mbed-OS that could be used to facilitate inter-process-communication (IPC).
That would be a starting point to make multi-process applications with Mbed-OS.