Make a cell separated diagonally in a table

UPDATE::

The Example in the Answer below works, But it may be from a result of some sort of BUG in Pages.

I did not save the test document when I did this until now.

It seems when you reopen the Document. It saves the result in the total cell of Table A, as text. i.e removes the formula.

So the REAL answer is NO. you cannot do this in Pages.

You CAN do what you want in Numbers

Original Answer

In the cell of Table (A) the table you want the sum; type a equals symbol. =

You will now get a the formula box. ^{(cell in table A)}

Type SUM(headerTitle) in front of the = with no spaces.

^{(cell in table A)}

The headerTitle being a Header Title from Table (B)'s Header Row or Header Column.

In this example I use Table B's Header Row title 'headRowTitle2'

^{(table B)}

(The Row in table B will highlight to show the cells Table A will be using)

Click the green tick button. It should now display the total you want.

^{(table A)}

Use the inspector to change the table cell formatting symbol.