JSP 814 states the following although I recognise single service headquarters may differ. I think the SCC has, for example, applied the same principle to reserve forces personnel as stated for regulars in 4.1.10 below, as people say ACP 20B has in the ATC/RAFAC.
4.1.9. Dual Appointments. CFAV may serve in more than one CF at the same time providing the Commanding Officers of the units are aware and in agreement.
4.1.10. Regular Forces. Regular Armed Forces personnel may not be enrolled as CFAV, although they may help at cadet units as ‘Service Helpers’ or similar.
4.1.11. Reserve Forces. Reserve Forces personnel may be enrolled as CFAV in addition to their Reserve commitment. However, WOs, NCOs and other ranks serving in the Reserve Forces are not permitted to hold a commissioned appointment in the CF. The implications of this are that an individual cannot serve as a commissioned or appointed officer in one organisation and hold a non-commissioned appointment in another. Instead of enrolling as CFAV, members of the Reserve Forces may help Cadet Forces as Service Helpers in their reservist rank.