Gridview - allow only one dropdown to be selected

Hi all,

I have a gridview that contains two dropdown lists, Session and Attendance,
along with some other columns. When the user makes changes and clicks the
Submit button, I need to somehow check each row to make sure that Session and
Attendance are not both selected. The user can only select one or the other.

So when the user clicks Submit, if any rows have both Session and Attendance
selected, a box would pop up saying "You may select Session or Attendance but
not both." A generic message is fine -- I don't need to tell them which
row(s) failed the check.

Any ideas how to implement this check?

Thanks,
Emily