What will be the equivalent voltage rating of two capacitors of different voltage rating connecting in parallel ?
And how does it differ in the series connection of two capacitors with two different voltage ratings?
You cannot apply a voltage higher than the lowest rated capacitor voltage to the string. If you have 3 capacitors, one rated for 16V, one rated for 35V, and one rated for 100V, you cannot apply more than 16V to the string. Realistically you should even derate the minimum voltage. I would not apply more than 12 volts to a 16 volt capacitor.
Generally speaking, when capacitors of equal voltage ratings are placed in series the voltages are added together. That being said, if the capacitors have different voltage/capacitance ratings then there is no good way to determine the overall voltage rating. The "weak point", so to speak, could be anywhere in the string depending on the order of the capacitors. (Thanks Neil for pointing this out)
When 2 capacitors are connected in parallel, the voltage rating will be the lower of the 2 values. e.g. a 10 V and a 16 V rated capacitor in parallel will have a maximum voltage rating of 10 Volts, as the voltage is the same across both capacitors, and you must not exceed the rating of either capacitors.
With capacitors in series, they will have the same charge (current/time) and as the voltage is equal to Q(charge)/Capacitance, then the higher the C, the lower the voltage. All this means is that the voltage developed across the capacitor in series will depend on its capacitance, and so it is safest to only have the lowest voltage rating across the series capacitors.
If they all have the same capacitance value, then you can use the addition of the rated voltages. I would also recommend, as real capacitors generally have poor tolerance, that you do not apply voltages at or near the rated value.