There is no problem having a Master µ-controller or Host PC "talk" to multiple '51s on a single serial port (RS232). The problem arises when they try to "talk back," when any one device tries to raise its Tx line to +12V while another device has its Tx line at -12V. The solution for me was to use MAX2332s (maybe its MAX3223, I'm not sure) with the driver outputs kept in the HiZ state. Then I developed the system such that no Slave µ-controller could initiat communication. They could only respond to a query from the host. That way there was never an issue of two devices trying to "talk" at the same time. I am assuming that RS232 would otherwise be an acceptable protocol for your application.