Given that current through CA = 1A, current through $C\text{'}A\text{'}=2A.$ Now if A is connected to $A\text{'}$ and B is connected  $B\text{'}$ Find currents through CA and  $C\text{'}A\text{'}$ respectively  

$E_1=(2+4)\times 1=6V,E_2=(2+4)\times 2=12V$ 

There will be no current in AB and  $A\text{'}B\text{'}$  because all these points are at the same potential. From D to A

$V_D+6-2I_1=V_A,but{V}_D=V_A\text{'}=0$

$\Rightarrow I_1=3A$

For outermost loop:  $6-12=2I_1+4(I_1-I_2)or{I}_2=6A$

Current through  $C\text{'}A\text{'}=I_1-I_2=3-6=-3A$