Unfortunately, immigration info on censuses does seem to be rather hit and miss in general.
Hopefully, the certs will have some good parental info that you can use to trace the girls in Scotland. It sure seems strange that they would both put Wright as names. Any births of children Maggie had w/b protected by privacy laws but it would certainly be interesting to know what she put as mmn for any she had.
When you are looking at passenger lists you might want to look for an Elizabeth and Margaret as well, assuming these w/b their full names. And variations on the spelling of McDougall, such as one l or MacD. I don't think Wright gets changed a lot so that spelling s/b OK.
PB