Tmmoris is right, you can be in 2 cadet organisations at once but I have had people try to tell me otherwise. Refer them to JSP 814 and then they can re-educate themselves.
In terms of carrying over rank, unfortunately I have some bad news.
When I was considering going to wellbeck, that question was asked. The head of the CCF at the college said that you would have to prove your worth to welbeck before holding a rank in the CCF.
So unfortunately my understanding is that you wouldn't be able to transfer your rank.
This was back in 2014/15 however so they might have relaxed there rules. But in my opinion from what I saw at the time I don't think they would have.
All the best anyway mate!