Fluent validation: either have both fields with value or both must be empty

I am trying to use fluent validation to validate the following rule.

Rule: Either both LocationLattitude and LocationLongitude must contain a value or they must both be empty. You cannot have one field with value and the other empty. Can someone help me out with this.

This is the code I have written:

RuleFor(ar => ar.LocationLattitude) .CasCade(CascadeMode.StopFirstFailure);

RuleFor(ar => ar.LocationLongitude) .CasCade(CascadeMode.StopFirstFailure);

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