setting multiple font name in .gtkrc-2.0

[50191914 ee cityu edu hk]

Hello, I try to set multiple family name in ~/.gtkrc-2.0.zh_TW.big5 as follow :
(I have LANG set to zh_TW.Big5)

style "gtk-default-zh-tw" {
        font_name = "MingLiU,Helvetica 10"
}
class "*" style "gtk-default-zh-tw"


However,it don't work and the gtk+ program (gtk-demo) seem just ignore
the font_name and use back "Sans"

however,it is ok if I just put only one font like  font_name ="Helvetica
10"

I have read the comments in fonts.c in function
pango_font_description_from_string :
 pango_font_description_from_string:
 @str: string representation of a font description.

 Creates a new font description from a string representation in the
 form "[FAMILY-LIST] [STYLE-OPTIONS] [SIZE]", where FAMILY-LIST is
 
  a comma separated list of families optionally terminated by a
  comma,
  STYLE_OPTIONS is a whitespace separated list of words where each
  WORD describes one of style, variant, weight, or stretch, and
  SIZE
  is an decimal number (size in points). 


and it seem that my format is right according to the comment