You could add another rule that would override the multiple-selector one for the desired element, but that is messier and bad practice.
I think the best practice is to have a single rule for a given selector, so all properties that affect it are gathered in one place. I wouldn't change the stylesheet just to do that, especially if there are only a couple of instances, but if you are separating out a selector, put the individual property declaration into an existing rule for the selector (if there is one).