I do believe scaring on one twin and not the other is fine. Since that is an external factor and not a blood one.
I have a set of twins where one is scared/torn wings and the other isn't.
Not sure about facial expressions XD I do know that Kisho has a beard and his brother doesn't.... and one has a leg down and the other hasn't. Also Kisho has a read streak in his hair and also a marking on his nose so that would count towards the 10%.