The conventional scheme used these days (noting that this subject has been discussed here many times) is to select a processor that has two serial ports on board. There are many of them now days. Look at the 8052.com link to the left called "Chips" or click here. If you look at the column UARTs you can see there are a number of parts from various manufacturers with multiple UARTs. There is also a similar chip list at the Keil web site.

Another alternative is to build a UART in software. There are application notes around that cover this subject as well. Here is a link to one at the Cygnal Web Site: http://www.cygnal.com/appnotes/an015.pdf.

It is also possible to connect an external UART to the bus of the 8051 but you will find this a more expensive solution. These days that external UART may cost you almost as much as your microcontroller. There are also some parts made by Maxim that give you a UART on the end of a three wire SPI bus. These convert from the SPI to normal async serial communications but once again they will add 3->5 US $ to the cost of your hardware. Some of the Maxim parts however do have the Rs232 driver/receivers built in. Here is link to the Maxim Selection.

I hope this information helps.
Michael Karas