Is it possible to horizontally split one column into two new columns in vertical connection (not adjacent) so that the lower/upper column width can be changed without changing the width of the other?

Also, that if there are two groups of people namely A & B. People in group A attended a company conference in September and people in group B attended the conference in December.

There are hundreds of them.

I placed the names of group A people in column "Conference Sep" and group B in column "Conference Dec" respectively, and sorted them in alphabetical order by their surnames.



I need to identify which people were not in either of these conferences. Is there any way in excel to automatically highlight the difference between these two columns?