Only when number of completed oscillation differs by an integer, the two pendulum will again begin to swing together.
Let longer length pendulum complete $n$ oscillation and shorter length pendulum complete $(n+1)$ oscillation, for the unision swinging, then $(n + 1){T_1} = n{T_2}$
$(n + 1) \times 2\pi \sqrt {\frac{{100}}{g}} = n \times 2\pi \sqrt {\frac{{121}}{g}} $$ \Rightarrow n = 10$

