for isotope 1, I1 and u1 is known, coupling constant is measured.
for isotope 2, I2 is known and u2 is unknown.
taking reference from the above lecture, it is difficult to calculate/predict.
However to make guess, coupling constant is independent of spin of nuclear level. it will depend on magnetic moment of nucleus and magnetic field due to electron distribution. Since two elemets are isotopes, they will have same no. of electrons /electronic distribution. Once couling constant is measured experimentally, magnetic moment can easily be calculated.
Please reply if i m correct.