I would first use the setup to measure a for the first isotope, from which, knowing I_1 and mu_1 I can retrieve the value B_J/J, which should be the same for different isotopes. Then, I measure a for the second isotope. Knowing B_J/J from the first isotope and I_2, I can retrieve mu_2. I am considering the option of applying an external magnetic field though… not sure if this would make sense.
Not sure if this is what was expected.