I may be off base -- but many effects are done with masks. To make two tone fonts you can convert the fonts to outline. This creates a bunch of filled objects. You can then move the filled outlines to a black/white mask where it masks everything except the "filled" area of the font. Once that is done you can then draw anything you want on the lower layer which includes a two-tone gradient and that is what you will see.
Note: You may be able to move the font to a mask layer without converting to outlines.
Note: It has been a while since I tried this in Illustrator but I have done it numerous times in PhotoShop.