mouse-face on two adjoin characters

[Masatake YAMATO]

Hi,

I'd like to insert adjoin characters responding to mouse cursor
separately. How can I do it? (I'm surprised that I cannot answer
this question by myself.)

I tried following code.

    ;; *scratch*
    (progn
      (insert (propertize "X" 'mouse-face 'highlight))
      (insert (propertize "Y" 'mouse-face 'highlight)))

But as explained in elisp info, XY responds to the mouse
cursor together: the both XY are highlighted when 
I move mouse cursor to X. I expected only X was highlighted.

Masatake YAMATO