Because someone else in the world can lie, steal and whatnot, that’s your excuse to do it too?

Can you name any CEO who secretly ran multiple companies without board not knowing about it?

I think you don’t know what CEOs do along with their legal fiduciary responsibilities + duties.

I can't give you an example of anyone who's done it secretly, but, Jack Dorsey (and several others) have done it openly. Why did nobody argue they shouldn't be doing that because they owed their full effort to one or another of their companies, I wonder? Why is it only a bad thing if a lowly worker does it?

Do you really think Twitter's and Square's boards responsible for companies worth $100B+ combined are not aware and have not discussed whether their appointed CEO is up to par to do this?

Can you name me any employer who hired a full-time employee and is happy to pay them full-time salary while the employee deceives them for financial gain and works part-time and/or outputs the minimum amount of work hoping not to get fired?

If you believe you can do more work, go freelancing/contracting/self-employed.

If both your employers are aware and okay with you working two jobs - it is totally morally okay. But I don't know lot of employers like that.