I also have the need to do this. In MS PowerPoint under Windows, I made suc a table before:
The formular in the sub-divisions of the top-left cell were put there in a very clumsy way, but in PowerPoint I can make those slashes. I was wondering if I can do that in iWork, or if there is a more advanced software which can make more complicated tables and then insert into iWork.
I assume you drew the lines in the corner cell of the table, that they are not actually part of the table itself. I also assume you placed text boxes that contained the three symbols, that they, too, are not part of the table. You can do the same in Numbers.
Yvan, You are correct that no cell merging is necessary, as long as the entire row is the same size. That is probably a safe bet.
I tried a set of graphics to make a line across the two cells as a background image. It did not work well. The size of the graphics and the size of the cells must be perfect for them to align and look like one solid line. I suppose if it was important enough someone might spend the time getting it perfect but that person is not me.