Are you making them red or green?
Cous cous is free on a green day. It's 1½ syns per 28g (cooked weight) on an original day.
Sausage meat will be ½syn per sausage on an original day but 4 syns per sausage on a green day.
I would say do them as original, cos you'll have more sausage meat than couscous, I'd imagine! Then weigh out your couscous once it's cooked and shared between all the eggs, it'll probably go a long way and be virtually no syns.
ps: And if you use the Morrisons Eat Smart pork sausages, they are syn free on original, which would make your scotch eggs even lower synned