I agree with Michael - especially as the 8051 UART has a mode specifically designed for this (Mode 3, or "multiprocessor" mode)

NB: a master-slave protocol doesn't necessarily mean that the same node always has to be the Master for all time - it isn't hard to devise a scheme to allow nodes to exchange master/slave roles.

Both of these principles can be found running on 8051s (and others) in vending machines all around the world!