I've seen a few names in Speyside in the form <'x' alias 'y'>, and 'x' is almost always a Highland clan surname. I am under the impression that in some cases these might go back to the aftermath of the Jacobite Rising of 1745/6, when many supporters of the Stewart cause are said to have changed their names to avoid the notice of the Hanoverian authorities who were trying to seek out and punish those whom they regarded as rebels. This (or these) MacDonald alias Grants just could be descendants of MacDonalds who had changed their name to Grant for a time, but were becoming more confident about using their real names. I have also noticed that when the <'x' alias 'y'> has been dropped, it is mostly 'x' which survives through later generations.
I don't think I have ever come across the <'x' alias 'y'> name being applied to someone who was illegitimate, or to a woman who had been married before. These have always been in the form <'x' or 'y'>.
I could, of course, be completely wrong, and I might find if I looked at more of them that the patterns are not as I have described. But at worst it's a possibility to bear in mind.