Can my students progress to study both AS & A Level IT 9626 and AS & A Level Computer Science 9618 in the same series?

Syllabuses 9626 and 9618 cannot be combined in the same sitting. The reasons why certain combinations of syllabus entries are not possible can vary by subject, but generally, this is because there is either similar or conflicting content between syllabuses. 

In the case of syllabuses 9626 and 9618, there is a considerable content overlap between syllabuses, including: Data types, security and integrity; compression; networks, network protocols and data transmission; monitoring and control technologies; system software; database concepts; algorithms (including pseudocode and programming); development life cycle, testing and maintenance. This list certainly is not exhaustive but does demonstrate why the two syllabuses have to be a disallowed combination.