In fact,every teaching contract in China that I've signed has either stipulated that I must first get permission from my employer before taking a second teaching post offered to me elsewhere, or that I am forbidden to teach elsewhere.
That means that FT's who have signed such contracts,and then secretly teach on the side are cheats.If they disagree with the clauses of their contracts,they should not sign on the dotted line.Not only are they cheating their principal employers,but also the taxman!