Two isolated charged metallic conductors of capacitance $C_1$ and  $C_2$ having charges $q_1$  and  $q_2$, respectively are connected to each other. After connection, there is 

Charge sharing happens when potential of the two conductors are different, i.e., when
$\frac{q_1}{C_1}\ne \frac{q_2}{C_2}\Rightarrow q_1{C}_2\ne q_2{C}_1$

Therefore, charge sharing (and hence loss in energy) will happen when 

 $q_1{C}_2\ne q_2{C}_1$