There are some concerns that need to be addressed to support a configuration where slot 0 is on an internal flash device, and slot 1 and scratch are on a larger external flash. This allows most of the internal flash to be used for code.
Although there is some support in the code, it hasn't been tested, so will need simulator support to test this.
The Zephyr port is hardcoded for a single flash device.
How to handle the write alignment being different between internal and external flash devices.
Addressing issues discovered by the above.