Generally speaking, this method can be used to start various other programs on the consoles (that is to say, you don't have to start the shell; it could be any program).
To do this for serial ports, start the appropriate serial driver (e.g. devc-ser8250), and redirect standard input, output, and error for each port (e.g. /dev/ser1, /dev/ser2). Then run the appropriate executable (in the background!) after the redirection.
The [+session] directive makes the program the session leader (as per POSIX) — this may not be necessary for arbitrary executables.