Sony stalk is not another serial, but analogue (resistive) input. See Sven's description on riocar.org. There is another serial port (inversed TTL levels, IIRC, search for Hugo's correction of my previous post), but it is already used for tuner.

There are, or rather were, multidrop (multipoint) serial interfaces (they were popular for connecting computer terminals and instrumentation once upon the time, approx. in the days of Cinderella ), but they require some kind of protocol, either SW (with addressed packets), or HW (with additional address lines).

There was talk here about defining our own multipoint serial protocol, possibly with an external demultiplexer (adapter to 'ordinary' RS-232). guys@empeg also said they had a design for it (probably for yet another secret OEM project). It kind of died....