Indeed, one transmitter - one wire. But you can connect more than one listener to that same wire, as long as there is enough power in there to drive them. Probably not the optimal way to do things, though.
Now I do wonder how they deal with that in Seatalk. It runs an actual bus over 2 wires (with multiple talkers and all).
Correct me if I'm wrong, but while the standard allows for multiple talkers it can only be done via a multiplexer... the multiplexer will act as a hub, aggregate the inbound datastreams and then output them to as many listeners as the multiplexer can support. Now I have to go do some research to verify this... I can't see how a serial connection could have more than one device outputting at one time though... Since it's really just a series of electrical impulses and there is no collision control on serial, it would just seem like a mess to any listening devices, wouldn't it?