Sending FF would work just fine. It is the handshaking that determines the time the data is valid from one side to other.

In the case of 2 microcontrollers tied port to port you would not need the 74LS07. The 8051 ports are open drain and so the lines share OK. You also can make it work simply with the one 8 bit port on each microcontroller plus the handshaking lines.
Michael