8051 has one hardware UART, but you can use any general I/O pins and/or interrupt pins to create additional UART(s) in software ..
Maybe you should consider this option before trying to connect external hardware UART ..
Here are more details on "bit bang":
**broken link removed**
"Bit-bang" serial I/O functions for the 8051.
These routines transmit and receive serial data using two general I/O pins, in 8 bit, No parity, 1 stop bit format. They are useful for performing serial I/O on 8051 derivatives not having an internal UART, or for implementing a second serial channel.