The method GradebookService.saveGradesAndComments allows you to save grades and comments for all students for a single gradebook item. Though this method was added to the GradebookService in 2.6, I believe that the only place this method is called is from the contrib assignment2 tool.
From the code perspective, call GradebookService.saveGradesAndComments to add the same comment for 2 students who do not have a grade comment yet. Only one of the comments will be saved.
If you are using assignment2, this is reproduced via the grade/feedback spreadsheet upload. If you try to upload a grade spreadsheet with identical comments for multiple students, only one comment will be updated. See http://jira.sakaiproject.org/browse/ASNN-584