To accomplish a split partition between onboard and external QSPI you will need to implement your own custom image_sha256() function to have awareness of the partition geometry, since the SHA calculation is currently done on contiguous addresses.
We support the NXP Kinetis K82 hardware and the K81 is compatible. The FRDM-K82F board we use for testing has an external QSPI with XIP support, which I have used successfully. However, I have not tried setting up a split boot partition. If you would like some consulting help with this feature please email firstname.lastname@example.org with some project details and we can quote this. If you would like to implement support and provide these changes upstream feel free to open a GitHub pull request.
David Garske, wolfSSL