The second period doesn't need to be "justified", it needs to be a situation "specified by the Secretary". In other words, there needs to be official guidance from the Department of Treasury or IRS saying that situation qualifies.
If your gain is large, it may be worthwhile going to a tax professional. When interviewing who to go to, you should specify that they may need to do research for situations that qualify as "Nonqualified Use", for purposes of the Section 121 Home Sale Exclusion.
