If two capacitors, each having capacitance C and breakdown voltage...

When two capacitors of capacitance $C$ are connected in parallel, the equivalent capacitance is the sum of their individual capacitances.

$C_{eq} = C + C = 2C$

In a parallel combination, the potential difference across each capacitor is the same and is equal to the potential difference across the combination. Therefore, the maximum voltage that can be applied across the combination without breaking down either capacitor is equal to the breakdown voltage of the individual capacitors.

Breakdown voltage of the combination $= V$