I did the Army co-op and Air Cadets at the same time. No one ever told me I couldn't do both. If you want advice comparing Cadets and the Reserves, I found the Reserves to be way more mature than Cadets.
I did the co-op with two fellow cadets, and they both quit Cadets because they enjoyed the Reserves more. Myself, I quit the Reserves after my co-op to help out with the leadership at my Air Cadet squadron (and for summer camp flight training). After not seeing these guys for a couple years, I invited them out to my Cadet graduation... it was like comparing babies to men to see the mature reservists stand amongst the cadets.
Anyway, why not try the co-op and stay in Cadets? After a few weeks of co-op, you'll know what you want. Base your decision on experience.