Similar to the ground rods comparison, the separate between Grid A and Grid B will be based on their maximum dimension (as well as the soil structure), as that drives the zone/sphere of influence of the soil potential.

I'm not aware of a guide/standard that is direct to your question, however we can consider the measurement process for the Fall of potential in IEEE Std 81. Essentially your grids can be 3-6x the maximum grid dimension(s) away from so that they do not conductively couple.

With substation grounding systems there may be shield wires or neutrals that metallically connect grid A and B so this isolation may not be possible.

In Power plants it is a requirement to interconnect the plant earth grid and the Switchyard earth grid. These grids may be separated by a distance of about 100m.

This question has arisen locally in the context of separated HV/LV earth systems in utility system. The standard answers I am aware of consider the relative size of both grids.

Since you wish to segregate based on the transferred potential via the earth itself, to achieve what you want, you must consider all connections to earth and possible parallel paths. The recommendation by @DBL-E is good, however you need to be considering using software modelling if you need a truly reliable answer.