There is actually another way to get a second serial port which I have used is inexpensive/simple systems. My parts selection was limited to those presently in inventory thus the following:

Use a MAX206 or whatever that has more receivers and transmitters in the same package and an 'HC157. Using the UART of the '51 and a routing pin along with software that permits only one serial link at a time via the "157, you enable one port at a time leaving the inputs/outputs in their non-communicating states. This does require that any serial attachments work on the 'speak only after spoken to' protocol.
Tom Burdick