What happens if two N-wells touch each other?
The DRC rule you are referring to is the separation of two wells at different potentials. DRC has no way of knowing what is the potential of the well (no connectivity information at this level), so if you have two separate NWELLS it will enforce this rule.

If you have two NWELLS at the same potential then you have two choices:


Directly abut them, so that these become a single NWELL
Separate them by the DRC rule minimum, so that the DRC rule checker does not give you an error and you don't run into further problems (see below)


Do note that on digital circuits, due to switching transients, two separate NWELLS will temporarily develop different potentials during normal operation. As the separation becomes an NPN region, you run the risk of triggering parasitic junctions on the design which can lead to some nasty outcomes.

This is not an issue if the well is a single unit, and it has enough well contacts to handle the potential disparities that develop across it.