When you have a merged cells between row 2 and 3 you cannot set 2 header rows : Ok.
When you have a 2 header rows table, you cannot merge a cell between row 2 and 3 : Ok.

But when you have a 2 header rows table, set a column header, you can now merge a cell between row 2 and 3, and remove the column : You have a table with normal and header cells merged !