A boundary line agreement is a legal remedy.  A boundary line agreement is only available as a solution to the property owners if certain elements are in place, otherwise it (the BLS) is not a valid legal remedy.  The major element is that the true location of the boundary is uncertain.  Some people add to this an element that the true location of the boundary is in dispute, the dispute over the true location being the factor that makes the true location uncertain.  If there are elements , tests are available for those elements.  For example, the uncertainty of location cannot be based on the fact that neither party ever bothered to make an attempt to find out the true location.  Another test of the element of uncertainty is that one party cannot know the true location while at the same time it is unknown to the other.  This may be an estoppels issue.