RX vs TX operation in Software UART
I'm not an electronics engineer, but I would go for using the TX operation as a software UART.

For an RX operation, buffering is needed, and interrupts are needed not to miss information. This is typically handled by a hardware UART.

For a TX operation, you only need to send information, which is happening when you want it (for receiving you don't know beforehand when data will be received).See hooskworks's comment (the right term is if the call can be blocked, it's easy by a software UART).

In case the UART speeds are equal, you can use one UART both for RX and TX. Even if the speeds are not equal and you know you don't receive anything while you send, you could switch speeds meanwhile probably.