Since the two isotopes are from the same element, the dipole field due to the electron cloud should still be the same. We can measure the parameter ‘a’ for both atoms with our set up (a_1 and a_2) and the ratio a_1/a_2 would then be (mu_1/mu_2)*(I_2/I_1). So we can calculate mu_2 by computing mu_1*(I_2/I_1)*(a_2/a_1).