It gets interesting when surname changes happen. Just as Lianne says, other names can be entered. It is especially important to track those if they were "official" surnames. Technically, I am a Fann, but not that many generations back, it becomes clear that an unmarried partner was where that name originated, so I might be considered a Porter, except it is by lineage only, not by usage.
Your scenario sounds like it may be one where both surnames may apply.