Hi Marco,
Which debug connection are you using? I have done a quick test using the Jlink connection and I'm able to debug both projects by adding this command to the debug Run Commands:
add-symbol-file C:\\Users\\b38285\\Documents\\MCUXpressoIDE_10.0.0_344\\workspace\\MK64FN1M0xxx12_Project_2\\Debug\\MK64FN1M0xxx12_Project_2.axf 0x80000

The address 0x80000 corresponds to the first flash section of the second project which was previously flashed to the MCU:

Hope it helps!
Best Regards,
Carlos Mendoza
Technical Support Engineer