If you are going to use the same icons both on mobile and desktop, then I don’t see a need for separating the set. The final file size of the fonts matters more than the number of icons. I would say that if the size of your WOFF or TTF font files is less than 60KB, stick with one font. Separating the font would have the drawback of additional HTTP requests. Ultimately, you would have to test and see which approach works best in your case.