Two non-conducting spheres of radii  $R_1$ and $R_2$ and carrying uniform volume charge densities $+\rho$and $-\rho$, respectively, are placed such that they partially overlap, as shown in the figure. At all points in the overlapping region, 
 

 

Let $C_{1}and{C}_2$  be the centers of the two spheres and $\overrightarrow{r}$  be position vector from $C_{1}and{C}_2$

Consider a point P in the overlapping region. Let $p_1$ is the position vector of P w, r, f $C_{1}and{\stackrel{⌢}{r}}_1$  is the unit vector along ${\overrightarrow{r}}_{1}Similarly,{\overrightarrow{r}}_2$  Is the position vector electric field at P is given by 
$\overrightarrow{E_Q}=\frac{1}{4\pi {\in }_0}[\frac{\frac{4}{3}\pi R^3{\rho }_1}{{\left(2R\right)}^2}i-\frac{\frac{4}{3}\pi R^3{\rho }_2}{R^2}]=\overrightarrow{O}$
$=\frac{\rho }{3{\in }_0}({\overrightarrow{r}}_1-{\overrightarrow{r}}_2)=\frac{\rho }{3{\in }_0}\overrightarrow{r}$
The electric field $\overrightarrow{E_p}$  is a constant in magnitude and has same direction  $\left(form{C}_{1}and{C}_2\right)$ at all points in the overlapping region . Since the electric field is non zero the potential is not constant