Thanks to hiroom2
1 /etc/default/grub
- Change GRUB terminal to console and ttyS0. This will provide one GRUB to a monitor display and serial console.
- Change linux kernel console to tty1 and ttyS0. This setting will be taken over to userland, and there will be two login prompt for tty1 and ttyS0.
GRUB_CMDLINE_LINUX="console=tty1 console=ttyS0,115200" GRUB_TERMINAL="console serial" GRUB_SERIAL_COMMAND="serial --speed=115200 --unit=0 --word=8 --parity=no --stop=1"