General rule: if you can draw LoS from the eyes of a model to any part of the body of the target, over, through, or under any intervening unit or obstacle, (BRB p. 10), you can shoot it. If you want to make sure, use a laser pointer.
So, it depends on the actual situation. However, in all likelihood, both the Captasus and the Orcs will be able to draw LoS to each other and thus able to shoot. With units facing each other, it is usually the case that, if you can see them, they can see you. But do not forget, only the first two ranks can fire anyway (except in case of Volley Fire), and you determine for each model (not unit) whether the target is within front arc, LoS, and range.
For cover, "the majority of the models in the target unit (or, when firing against a single model, more than half of the target model)" (BRB p.41) must be obscured.
More than half of the Captasus should be obscured - he benefits from hard cover. It is quite possible that the Captasus, because of the elevated hight of his eyes, could see the majority of the target unit. In which case, they would not benefit from any cover.
