If the communication to let's say two slave devices does not need to occur in parallel, you could use GPIO-driven transistors to select the device, you wish to talk to by enabling only the RxD and TxD pins of the current device. Requires at least two transistors and possibly resistors though.

If the communication to the several UART slave devices does not need to occur in parallel, you could share the one available UART port between them. You could use transistors to enable only the RxD/TxD connections to the device you currently wish to talk to. These transistors can be controlled by other Raspberry Pi GPIO pins.