Shared neutral with two separate breakers
As long as the ungrounded  (hot) conductors of the circuit are fed by different legs of the service,  then there's no problem with the wiring. According to modern codes, the breakers need to at least have a handle tie. However, since the installation presumably predates the code, it does not have to be changed (unless you're doing work that requires it).

If you want to install AFCI breakers, you'll simply have to replace the two breakers with a double pole AFCI breaker.