I have tried it on a Mega65 with the C64 core, unfortunately save didn't work to the disk image (stuck at "saving") and load did not work from the cartridge image ("save bank full, erasing") :(
I have tried it a few more times and found that the cause of the problem was that mounting the crt image unmounted the savedisk image.
Mounting the crt image first and mounting the save disk right after have resolved the issue: saving and loading using the savedisk image was successful.
