Most bonsai tools have convex faces so careless abrasion could flatten the ends and result in a gap between the cutting ends. If you are confident enough in your freehand sharpening skills you can use a good waterstone or some wet/dry silicon carbide sandpaper and sharpen the beveled side down. After sufficient sharpening you can remove the wire edge or burr. It would be imperative that you sharpen each cutting end evenly though, otherwise they may not meet up when closing the cutter. In any case sharpening concave cutters or similar tools are not easy. If the tools are really worn down I probably would just not put in the effort. What kind of sharpening tools do you have?
Also here is an article I found on sharpening bonsai tools:
https://www.evergreengardenworks.com/sharpen.htm