If the nuclei under consideration are not isochronous (i.e. they do not have the same chemical shift) then they are neither chemically nor magnetically equivalent. All chemically or magnetically equivalent nuclei are isochronous however, isochronous nuclei need not be chemically or magnetically equivalent as their chemical shifts may be fortuitously identical.
If the two nuclei being considered are isochronous, one should consider how they couple to a third magnetic nucleus in the molecule which is not equivalent to them. If the coupling to the third magnetic nucleus is different for each of the nuclei being considered, then the nuclei are chemically but not magnetically equivalent to one another. If the coupling to the third magnetic nucleus is identical (and this is true for every magnetic nucleus other than those being considered) then the two nuclei are both magnetically and chemically equivalent.
This is illustrated in the figure below. (The examples from Harris' book were used here.) In the molecule on the left, the two protons are both chemically and magnetically equivalent as they both have the same coupling with the fluorine. In the molecule on the right, the two protons are chemically equivalent but not magnetically equivalent as they do not have the same coupling to each of the fluorines. Similarly, the two fluorines are chemically equivalent but not magnetically equivalent as they do not have the same coupling to each of the protons.
