Hi Elana,
I know this post is old but it's given me hope! I have the opposite problem, I need my group headings to go across both columns.
Your problem may have been that you had the salesperson name in the page header where I think you want to put it in the group header.
If you have done this and the group header still crosses both columns, please let me know - this problem has plagued me for months!
-Sean