I know that absolute event horizons are complicated and depend on the details of the metric because of their teleological nature. But I assume that an isolated gravitational singularity of mass $M$ with no nearby matter is fairly generically surrounded by trapped null surfaces out to an apparent horizon of radius approximately $2GM$ (as seen by a faraway observer), since it should locally "look like" a regular Schwarzchild black hole.
But this would seem to imply that apparent horizons can be nested. For example, consider two gravitational singularities with masses $M$ and $m$ separated by a distance (as viewed from a faraway observer) in between $2Gm$ and $2GM$:
What would a faraway observer observe a light ray do if it were emitted from a point $x$ in between the singularities and within both apparent horizons? If it's on a trapped null surface centered at $M$, then it should have to move to the left, but if it's also on a trapped null surface centered at $m$, then it should have to move to the right. (In particular, if $M$ is so far away that its tidal forces are negligible, then light rays within the inner apparent horizon should locally "not know it's there.")
One possible resolution I could imagine is that the light way moves to the left as seen from a faraway observer, but $m$ moves toward $M$ even more quickly and gains ground on the light ray, so that the ray moves to the right relative to $m$. (In this case the observer would observe $m$ appearing to move faster than light, but I think that's okay because he's very far away, so local Lorentz invariance isn't violated.) But what if the singularities begin at relative rest (as viewed from a faraway observer)?