@Lazar9 I know it feels weird, but it's actually good for you as a host, to have 2 reviews from the guest in a case like this, as long as the reviews are good ones. As a host, it will be evident to future guests that your extended guest liked your place enough to want to extend the booking. So for the second review, your guest can hopefully say something like "Lazar's place is so nice, I decided to stay for another week." They don't need to write all about the place like they hopefully will do the first time.
And for your review of the guest, you could go into some detail on the first review, like they were friendly, clean, followed house rules (assuming they were good guests, otherwise I don't think you would have agreed to let them extend?) whatever, in the second review, you could just say "Glad this guest decided to stay on for another week." You don't need to write a second review, but that might encourage the guest to leave 2 reviews, which is good for you as far as "review rate" for Superhost.
Or you could just not leave a review for the first booking and write one at the end of the second. Review rate is the % of guests who review you, but you don't get rated on the % of guests you leave a review for.