The time period of a pendulum is given by $T = 2\pi \sqrt{\frac{L}{g}}$. Let $n_1$ and $n_2$ be integers for the number of vibrations. For them to be in phase, $n_1 T_1 = n_2 T_2$. Substituting the values: $n_1 \times 2\pi \sqrt{\frac{1.21}{g}} = n_2 \times 2\pi \sqrt{\frac{1.00}{g}}$. This simplifies to $n_1 \times 1.1 = n_2 \times 1.0$, or $\frac{n_2}{n_1} = \frac{1.1}{1.0} = \frac{11}{10}$. Thus, after 11 oscillations of the shorter pendulum, they will be in phase.