In the circuit shown in figure initially key K₁ is closed and key K₂ is open. Then, K₁ is opened and K₂ is closed (order is important). 

[Take Q'₁ and Q'₂ as charges on C₁ and C₂ and V₁ and V₂ as voltage, respectively.] 

Then,

The charge stored by capacitor C₁ gets redistributed between C₁ and C₂ till their potentials become same, i.e. V₂ = V₁. By law of conservation of charge, the charge stored in capacitor C₁ when key K₁ is closed and key K₂ is open is equal to sum of charges on capacitors C₁ and C₂ when K₁ is opened and K₂ is closed, i.e., Q'₁ + Q'₂ =Q.