Thanks for your help and sorry for not replying sooner.
I had seen it, but could not reproduce it at time of posting, but I finally
succeed!
The answer is actually quite curious. It seems to be a "bug" actually.
In order to "see" the text one over the other, you put text in A1 (that is
longer than colum width), you leave B1 blank, you write something in C1 and
align right. If the text in C1 is long enough to reach A1, it will go over
and you will see the content of A1 and C1 overlaped.
Not quite scientific... But efficient for some visual effect
Thank a lot for your time.