The effective spring constant of two spring system as shown in figure will be 

When external force is applied, one spring gets extended and another one gets contracted by the same distance hence force due to two springs act in same direction. 
i.e.  $\mathrm{F}={\mathrm{F}}_1+{\mathrm{F}}_2\Rightarrow -\mathrm{kx}=-{\mathrm{k}}_1\mathrm{x}-{\mathrm{k}}_2\mathrm{x}\Rightarrow \mathrm{k}={\mathrm{k}}_1+{\mathrm{k}}_2$