RS-485 is NOT full duplex. All modern 8051's have a double receive buffer. The issue with handling 256kb is not the UART but your receive routine, at 256 kb you will have about 40 uSec per byte, even reasonable - not even perfect - assembler code can produce an ISR handling this with a 12Mhz clock. Of course if you go for a 6 or 4 clock uC and a higher oscillator frequency you can do many things between every interrupt. Re receive buffer: let your ISR write to a circular buffer. Re 2 UARTS, you can implement a software UART for the slow line and use a 1 UART plus CAN controller.

Have fun and let us know if you mean RS-422.

Erik