Well, they could have been born Wright, say, then their mother married a man called McDougall (or the other way round) and they also were known by their step-father's surname. Or their parents may have been unmarried, so the girls were registered in their mother's name but knew their father and also used his name.
If you know Maggie and Lizzie's approx date of birth, you may be able to find them on Scotlands People, both under either Wright or McDougal(l). I shouldn't worry about the exact spelling, many more people were unable to read and write in 19th century, so would just say their name and not know or check the spelling.
