From the measurement of the hyperfine splitting of the first nucleus one finds the hyperfine constant a. Since the magnetic dipole moment and the nuclear spin are known one can find the constant \frac{B_J}{J} = \frac{aI}{\mu}
which depends on the electron cloud which we assume doesn’t change for the different isotopes. Now for the other isotope we then know this constant and the nuclear spin so after measuring the hyperfine splitting to find a one can extract the magnetic dipole moment from this.