Hi Omar,
I am no expert with 8051's but when I needed to communicate with 3 different equipments using RS232 I took the simpler way out i.e. I used a SCC2681 duart to add two extra RS232 ports to the 8051.
Later I realised that the cost of SCC2681 was much more that two AT89C2051 chips. So I replaced the SCC2681 with them(space was not a criteria for me). I Only had to write a small code for serial to parallel conversion and for hand shaking with the main uC.

Jawahar