The forms are actually XML files saved in the library, so you can't actually change the data in one file directly from another file. However, if you promote the properties that you want to update and check the box that makes them editable in the library, then that makes the data in the form get copied into the library itself (this is actually duplicate data - the data in the library is not the form data, although they will always match). Once these fields are promoted and editable in the respective libraries, then it's easy to make a workflow that copies the data from one to the other as long as you have a unique identifier to associate the two forms.