If the separation distance between two charges is increased, their...

The electric potential energy of a system of two charges $q_1$ and $q_2$ separated by a distance $r$ is given by:

$U = \dfrac{1}{4\pi\epsilon_0} \dfrac{q_1 q_2}{r}$

If the charges have the same sign, $q_1 q_2 > 0$. As $r$ increases, $U$ decreases.

If the charges have opposite signs, $q_1 q_2 < 0$. As $r$ increases, $U$ becomes less negative, meaning $U$ increases.

Thus, the electric potential energy may increase or decrease depending on the nature of the charges.