This video shows that it is possible to use a C128's auto-booting feature to install auto-booting cartridge code into memory, and then switch to C64 mode, which executes the auto-booting cartridge code to kick off an auto-boot of C64 OS in C64-mode.
In this video a uIEC/SD (SD2IEC-compatible storage device) is being used, powered externally via USB, and has a special file called bootsect.128 in the root directory of partition 1. Powering by USB is necessary to make this work so that the drive can be fully powered on and ready before the C128 is turned on. It was also necessary to remove the reset jumper from the bottom of the uIEC's Deluxe Daughterboard.
