If on the concentric hollow spheres of radii  $r$ and $R\left(>r\right)$  the charge  $Q$ is distributed such that their surface densities are same then the potential at their common centre is

$$\begin{gathered} Let say charge q_1 is on inner sphere and charge q_2 is on outer sphere \\ {q}_1+q_2=Q and \frac{q_1}{4\pi r^2}=\frac{q_2}{4\pi R^2} \left(given\right) \\ {q}_1=\frac{Q{r}^2}{R^2+r^2} and q_2=\frac{Q{R}^2}{R^2+r^2} \\ Potential at common centre=V=V_1+V_2 \\ =\frac{1}{4\pi {\in }_0}\left[\frac{Q{r}^2}{\left(R^2+r^2\right)r}+\frac{Q{R}^2}{\left(R^2+r^2\right)R}\right]=\frac{Q\left(R+r\right)}{4\pi {\in }_0\left(R^2+r^2\right)}\left(\because Potential inside shell = potential on the shell\right) \end{gathered}$$