For comparisons, perhaps look at the fact that both girls were plagued by the death of numerous family members. Also, a place to start might be that the two were avid recorders of what things were happening around them, specifically during the Civil War. Both are intelligent, female writers in a time when that wasn't the norm.
By contrast, one girl supported the Union and one the Confederacy. What might their different views on the government, slavery, and the war, itself, be? If they were put together in a room, what might they talk about? About how much would they agree or disagree?