If we first measure the hyperfine coupling constant a we would know the perturbing hamiltonian. The perturbing hamiltonian equals the moment operator times the field operator. Since we are dealing with two isotopes the number of electrons is the same and we could approximate the field operator as the same for both isotopes. It is now possible to calculate the field operator using the perturbing hamiltonian from the first isotope and the magnetic moment calculated from the given data. Since we now have calculated the magnetic field, we can measure the perturbing hamiltonian of the second isotope. Because the field is the same we now know the magnetic moment operator of the second isotope. Out of this we can calculate the magnetic moment of the second isotope.