I need to split or divide one cell so it can hold 2 text values divided. Something like:
In the top left cell create 2x2 table with invisible borders. Then in this nested table add diagonal border to top left and bottom right cells:
Here it looks nice, but in real life it sometimes could be very ugly. But, this is the most reliable way from my point of view. You can write the text of any length and use any font size. Also, shapes (as suggested in original answer) are sometimes misaligned when you change the paper size - I don't use them in any of my documents.
Okay, 2 reasons why the accepted answer is unperfected, and why I post mine.
You need to modify line again if you change the width of the cell.
Original screenshot of the comments:
I found an easy way to do this. Click the
Draw Table button, and your mouse become a pencil, then draw a line to split the cell. Voila.