Serial connection two devices two power supplies
The 3.3V zener diodes are connected backwards. The anode should be connected to ground and the cathode (banded end) to the GPIO signal. Zener diodes work in reverse bias...as you have them connected now they are forward biased and will clamp the signals to about 0.7V.

The zener diode on the line for data transmitted at 3.3V to a 5V input doesn't do any good. You need to increase that signal voltage to a proper 5V logic level. The best thing to do would be to get rid of the zener diodes altogether and use a proper logic level converter.

Also, you really should have resistors between GPIO output pins and the zener diodes, to limit the current through the zeners when the GPIO pin is at 5V.