Yes you can and its been done numerous times. One is a youth organization and completely voluntary, the other we all know what it is. I've seen it done and did it myself ages ago.
The issue that arises is when member gets their priorities mixed up and puts the youth organization ahead of their military obligations.
You can do both, but must realize that the deconfliction of scheduling and what CoC you answer and when must be clearly stated and understood.
The CF is the priority, cadets second; period. If the member can't wrap their head around that, it's better to just put off getting in the reserves until you hit 19 and you have to quit cadets anyways.
Regards