To set the default font in Emacs, use
(add-to-list 'default-frame-alist '(font . "-adobe-courier-bold-r-normal--20-140-100-100-m-110-iso8859-1"))
The command set-default-font sets the font for the first
frame created, but does not set the font for any subsequent frames, so
if you use
(set-default-font "-adobe-courier-bold-r-normal--20-140-100-100-m-110-iso8859-1")
the second frame will come out in a different font.