From what I recall of the GM datastream I played with , the PCM (powertrain control module -- I think they got NASA to name that one) would periodically emit a packet of data containing, among other things, vehicle speed. A lot of the relevant data was pushed, and you could just listen to the wire. Things like fuel maps were more complicated and required a bidirectional protocol. (Caveat: I only spent a few weekends playing with this stuff...) I have no idea if OBDII resembles this, but my guess is that it does.

If you wanted to multiplex the serial line, you could hook a pair of analog switches to one of the serial port control lines (assuming they exit the wiring harness)... then you just assert/deassert e.g. DTR or CTS in software to switch.