Your @charset can only appear once in a stylesheet and when you import the other, now its there twice. Secondly, your @charset and @import statements need to be on the same line if in the same file. I typically take my font stylesheet css and place it in the top of the main stylesheet, makes it a bit easier to work with. Other option would be to create a new stylesheet in the Site Manager and use the Head HTML webpart to place it on your page. I use this method quite a bit as well too.