From exercise #12 on they return to be for two hands. As you can notice, exercises #12 and #13 are mirrors, in which right hand and left hand reverse their roles. All in all, this exercise provides for the execution of the five notes scale, from C to G, ascending and descending, firstly with the right hand (ex. #12) and then with the left one (ex. #13).
I wouldn’t expatiate too much about it, because I think from this moment on (unless particular or innovatory cases explanations) only good practice is useful.