When CHECKING eyes with a penlight, I always thought you cover the opposite eye to get the most reaction to light. Darkness to abrupt penlight will immediately show you just how PEARRL those eyes are
When COVERING eyes due to an eye injury, the DOT says to cover both because if one eye can see, it's likely to look left and right. This is just going to aggravate the injured eye, as both eyes move at the same time.