It is nowhere near a core activity though. Other cadet organisations can just be a better fit to a person’s real passions.
As for the original question, I cannot see anything in JSP814 which would preclude a staff cadet in the ATC also serving as a CFAV in the SCC. Separately, CFAVs and Cadets can be members of more than one CF at the same time if the commanding officers are aware and agree. At the very minimum I would expect that to apply to general membership of two CFs if a cadet in one and a CFAV in the other. Personally, I do not see it being a conflict either, and may actually be good personal development.
I expect the HQs of the 2 organisations to panic about it though.