Hi,
There are external UART devices.
********
Without external UARTs you may use:
* diodes + pullup to create "wired AND"
* AND gates
* three state buffers
* MUXes (digital as well as analog)
* PLDs, FPGA
* different microcontroller
...
You see there are plenty solutions you may choose..
Btw: for more detailed answers you should post a sketch about your application. Signal flow with informations like usage timing, baud rates, transmitted data information, micrcontroller type...and so on. Even hand drawn is better than text.
Klaus