Patrick said ” If you do not have a supervisor chip, your 8051 will have to stop whatever he is doing if a byte is sent for transmission in order to read (and accept) the byte. You will have to queue (read: buffer) your data if you do not want to treat your data immediately, but that is a plain (not always simple) buffer mechanism.”
Patrick said “So why should both your devices be tied up communicating?”

They are not tied up,
FIFOs jobs is take care of queuing so 8051s do not need to be interrupted because data of other 8051 goes directly to a FIFO. The only exception is when a 8051 has sent many packets without ACK or NAK response, in that situation the sender 8051 may want to interrupt the other to make sure the receiver chip is still conscious and reset it if it was not. so 8051s do not have to stop.

Patrick said
“AN example:
If you are sending data to your printer, does he stop printing to receive the data? I don't think so.. It is receiving data into it's buffer, and yet it is printing.(and very likely using a parallel centronics or IEEE1284 I/F...)”

Centronics needs hand shaking so the 8051 should stop for handshaking.

Regards,
Ebrahim Vakilpour