Moneywise, Deng's two daughters would get 1/3 of company's shares (probably $1.5-2Billion each one), there is no need for Murdoch to divorce Deng for the sake of protecting Deng and her daughters financial interests.
"Mr. Murdoch’s six children have equal shares in the company through a family trust, but only Prudence, Lachlan, James and Elisabeth have the right to appoint trustees for the trust, which will give them control over the company when Mr. Murdoch dies."
http://finance.yahoo.com/news/14-years-murdoch-files-divorce-172745889.html