How do we find the breakdown voltage of capacitors connected in series, if their individual breakdown voltages are known??

Breakdown voltage is the voltage at which the insulating dielectric material between the capacitor plates breaks down and conducts electricity.

In series: Breakdown voltage = 2 * Vb
(but only if the capacitances are equal!)

In parallel: Breakdown voltage = Vb

...where Vb is the breakdown voltage of the capacitor, and assuming Vb is the same for both capacitors.