Connecting a half duplex and a full duplex (2 slaves) to a MCU's UART
That is why the question was, how can the two slaves be connected to the same UART and be controlled by the MCU. 


By buying a MCU with at least two UARTs.

You can either use your MCU to do the following

                                ---- Slave 1
                               /
ATMega128 ---&gt; additional MCU &lt;
                               \
                                ---- Slave 2


i.e. the additional MCU gets a command from your Atmega to switch the output, e.g. using an external switch, or, by assigning the UART to other pins.

Or, really, you can ditch the totally overpriced, underpowered ATMega128 and simply use a microcontroller that has enough UARTs on its own. There's very many cheaper microcontrollers. A lot of these are mature ARM Cortex-M0 processors, which, I'm taking a guess here, will even be easier for you to write code for (being way faster, and having much better C and C++ (and generally, any systems language) compilers).