As we are talking about the same element, the electronic angular momentum and magnetic field are the same. The hyperfine parameters a_1,a_2 are thus related as follows:
\frac{a_1}{a_2} = \frac{\mu_1}{\mu_2}\frac{I_2}{I_1}
This can be derived by dividing the expressions of a_1 and a_2.
Experimentally determining a_2 would thus give \mu_2