SELECTED ANSWER
replied on November 11, 2014
You can't change templates on the fly per document, templates are global.
3 fields required:
"Comments" (multivalue)
"Add Comment"
"Engineers Assigned" (multivalue)
However if I was approached with this scenario I'd make the comment multi-value field read only. When they add a new comment you can use initiator to determine who added the note and then put that note in the comments multi-value field with "(%initiator) - (%note)" so that it is prefaced with it. Then you blank out the add comment field.
The engineer multivalue field has the names of the engineers in it. When someone edits the document, have it populate the comments field then retrieve the values of the comment & engineer fields. You then use a nested loop to look for each "$(%engineer) -" in the comments field. This way it only matches the engineer's name if it's at the start of the comment. If the nested loop DOES NOT find a match, end. This way it only goes past these nested loops if both engineers have a comment.