Two identical capacitors are joined in parallel, charged to a potential V and then separated and connected in series, i.e., negative plate of first is connected to positive plate of second:

the potential difference between the free plates become 2 V

When they are connected in parallel, each capacitor will have a potential V. When they are joined in series having each of potential V, then total potential across the two extreme plates of the capacitor is 

From Charge conservation $CV + CV = 2CV'$

$V'= 2V$