As ${T_1} > {T_2}$==> ${n_1} > {n_2}$ giving ${n_1} - {n_2} = 6$
The beat frequency of $6$ will remain fixed when
$(i)$ ${n_1}$ remains same but ${n_2}$ is increased to a new value $({n_2}^\prime - {n_2} = 12)$ by increasing tension ${T_2}$.
$(ii)$ ${n_2}$ remains same but ${n_1}$ is decreased to a new value $({n_1} - {n_1}' = 12)$ by decreasing tension ${T_1}$.