The NEC PC-8201 had three serial interface ports on the back, "RS-232," "SIO1" and "SIO2." These were all driven by a single IM6402 USART, switched between each physical port via bits 7 and 6 of IO port $90. Details are given in §3.1.8 (p. 3-10) of the NEC PC-8201 Service Manual (PDF).
This allowed a user to plug in multiple devices simultaneously and very quickly switch between them, quickly enough that a user might feel all devices were working at the same time, though of course actually only one device would be accessed at any particular instant. It saved on cost, took up fewer of the 256 I/O ports, and also saved considerable board space on a small machine (the IM6402 was a 40-pin chip).
What other 8-bit microcomputer systems used this technique, where a single controller (such as a UART) that cannot control more than one device at a time would be switched between different devices? (The devices may be on-board or external devices, and, if external, the switching logic may be onboard or in a separate "port expander" unit.) Ideal answers will include the user's view of how the ports were used, an explanation of why the designers implemented the system this way, it, a techincal description of the multiplexing scheme, and references.
Note that I'm not asking about external multi-device bus systems such as Atari SIO, but only systems that appeared to the user to have separate external ports where the devices did not share the external port with other devices connected at the same time. (Or, for internal muliplexing, where the device itself was not "aware" that was sharing a controller with other devices.)