They can be different, but your main NOC is the one they will evaluate to see if you meet the skilled work requirement (e.g. 1 y ear of continuous full-time employment in a skilled NOC) for express entry eligibiltiy and program eligibility. If you have at least 1 year of full-time continuous employment in Canada, I think if you choose that NOC as your main occupation, you will be invited to apply for CEC class. But if you have less than a year, you won't be eligible, then you should designate your other NOC as the main.