Hi,
So you want an 8051 MCU with 2 UART's. There are many types of this sort of MCU's. You can look e.g. on this DS83C520 **broken link removed**. It has 2 lines Rx and Tx.
If you would like more help ask
There are 2 devices from NXP, the LPC952 and the LPC954 that come with 2 UARTs.
THe core is a 2-cycle 8051 core and the internal oscillator is good enough for your UARTs, no need for an external oscillator.
Bob
Products for professional embedded solutions 16- and 32-bit controllers **broken link removed**
what about using a simple tristate buffer than going for microcontroller with two Uarts. 74244 is enough for this, but there should be one master controller (either pc or uc) to do this