Is anybody know what font family can be used in order
to let each character(eg. W and a) occupy the same
space in xsl-fo?

Do you mean you want to have two characters printed on top
of each other? For *every* pair of characters? That's weird.
And I don't think this has something to do with font families,
apart from the problem that support for specific font families
is processor specific.

I guess you are trying a problem the wrong way, heading down
a dead end.

Please describe your original problem, and also state the
processor you are using, including its release number.