Historically the Spanish had them under control for 1 reason; in-fighting.
The Abbadids expanded north at the expense of the Aftasids whenever the Spanish kingdoms ended up fighting them, and at the expense of much of their own wealth. By the time the Abbadids and Spanish had wiped out the Aftasids and developed a mutual border the Abbadids had exhausted much of the wealth they had on hand which they'd brought with them from Arabia. This indirectly led to a powerful Portugal and Leon once the now expanded and strengthened Spanish kingdoms had acquired half of the Aftasids land (there's some complicated steps in between but that's the gist of it). They then tore the Abbadids apart, albeit slowly, step by step, all the way to Gibraltar.
However, in the game the Aftasids have two very powerful duchies, and the structure of the CB mechanics in the game make it easier for them to maintain their specific territorial boundaries. Also as they're one of the strongest and the AI isn't particularly good at small-scale opportunism the Abbadids rarely play their historical role here.
In the east the situation is just a mess but that's historical. The solid Aftasid block though, which tends to have the Abbadids expand smoothly around them to meet christendom in the east, results in a rather a-historical balance of power where those two work together (especially to defend v holy wars) and tend to contribute greatly to the dismantling of the Spanish kingdoms, when historically the two powers almost never assisted each other.
