There is no resolution information in the JPEG standard. I think you are talking about JFIF/JPEG information, which technically should not exist in an EXIF/JPEG image. However, some image utilities rely on this (and many write JFIF when EXIF already exists), so you can do this if you want. ExifTool will not create the JFIF segment unless you specifically write the JFIF information. ie)

-jfif:Xresolution=300 -jfif:Yresolution=300

- Phil