If one wants to have the boot code send the initial stuff to both the serial and VGA, one should use
echo '-Dh -S115200' > /boot.config
but the option -Dh does not seem to be mentioned in boot.config(5) ?
: A valid format of this file is to put BIOS drive number, a controller
: type, a unit number, a partition, a kernel file name, and any other valid
: boot(8) option <...>
Have you checked boot(8) man page?
Ok to close the ticket?