I don't know if it counts because there was only 2 of them, but Groosham Grange by Anthony Horowitz (we started reading it in school but then I moved schools so never got to finish it).
A longer series I bought as it came out (with pocket money mind you because I grew up as each book was releasing) was The Power of Five pentalogy, incidentally also by Anthony Horowitz.
I don't know if it counts because there was only 2 of them, but Groosham Grange by Anthony Horowitz (we started reading it in school but then I moved schools so never got to finish it).
A longer series I bought as it came out (with pocket money mind you because I grew up as each book was releasing) was The Power of Five pentalogy, incidentally also by Anthony Horowitz.