If you do not want to switch these variables,
because you still want output on the default UART
you will have to access the device driver.
Unfortunately drivers/serial/serial-uclass.c
does not yet export functions for this.
But _serial_putc(), __serial_getc(), and __serial_tstc()
should give you an idea how this is done.That would indeed be useful. I was facing a similar issue of having to
initialize multiple UARTs, and ended up probing additional instances
manually from the board file using the respective DM functions
(unreleased code).
Not pretty but it worked.
I'll be happy to test such a patch if created.