Back to back Schottky diodes between ground connections
What purpose would they serve if the ground places are already separated? No coupling should occur if the designer properly managed to split the planes.


And that's part of the problem. Many mixed-signal devices (ADC, DAC, etc.) will misbehave or become damaged if the two grounds are at too different potentials. The diodes prevent them from drifting apart too much.

Obviously if the grounds are supposed to be at completely different potentials (e.g. isolation) then the diodes should not be placed.