First, you need to carify (mainly for youself, but you can do it here, if you are not afraid of some criticism) what exactly are your requirements - baudrate, half- or fullduplex, other processing time needed, priorities of accessing peripherals in your design. You might e.g. consider moving the LCD and barcode scanner to the intended SW UART (which might have lower priority and/or might tolerate ocassional error) and leave the HW UART for communication with PC - but it's all upon you. You should be also aware, that SW UARTS usually don't perform multiple sampling per bit as HW UARTS mostly do, so it cannot filter out sparse noise and spikes - if it matters at all.
But I doubt you will be able to come up with anything reasonable in C. There is an appnote at Atmel's website on "second asynchronous interface" (originally by MHS) with nicely devised fullduplex SW UART at 1200/4800/9600 Baud, using the same timer as the HW UART running at 1200 Baud (at 11.0592MHz clock, so you arel be able to do twice more). It is all-assembler, and takes up around 50-60% of processing time - think about it.

Jan Waclawek