hi,

if I understand right, you have connected P1 of both MCU together, right?

First of all, let me write numbers with binary notation:

                               D7      D0
1) RD+ sets data line to 0xF0:  11110000b ; o.k.
2) C51 sets data line to 0x80:  10000000b ; o.k.
3) RD+ sets data line to 0x40:  01000000b ; false

I am surprised that after all you have read 0x80 from common data bus. In fact, it shoul be zero (00000000b) because both MCU have put all data lines to zero. At step 3, RD+ is not able to set high level on D6 because it is shorted to ground with C51.

Regards,
Oleg