There's nothing poisonous in it, can't do any harm. It is not as weird as some other combinations I have heard of such as Pineapple Juice and Coke mixed (Muddy Puddle), Sausages and Marmalade, Stewed Apricots and chocolate Sauce. I would think Fish and Cous Cous is OK.
We often have this. I've done it with salmon, mackerel and white fish. Nice with peppers and sultanas in the couscous before you add the water or stock. Stir in some lemon juice when you "fluff" it as well; goes really nicely with the fish.