With one major british airline I know of, you have to get permission from the employer to hold a second job or declare an outside interest in a charity, etc. If denied by the employer you would be subject to a disciplinary if you continue with the other work, and if permitted to take on the other job, the individual is responsible for their health and welfare, and the agreement stands that their primary employment (with the airline) is of higher importance and the staff member must act responsibly. If at any point they suffer from fatigue or ill health as a result, or if its intefering with the airline contract of employment or safe operation, the situation is reassessed and a decision is made to either cease or reduce one or other employment.