By Diane Bartz

WASHINGTON (Reuters) - Health insurer Anthem Inc <ANTM.N>, which is fighting the government to save its merger with rival Cigna Corp <CI.N>, asked a judge on Tuesday to decide on the deal by the end of the year and to split its case off from the government's challenge of a second merger of insurance companies.

The Justice Department filed lawsuits on July 21 asking a federal court to stop Anthem's deal for Cigna as well as Aetna Inc's <AET.N> planned acquisition of Humana Inc <HUM.N>. This means that a judge will decide if the deals can go forward.

Anthem asked on Tuesday for its case to be tried separately from Aetna's deal for Humana, arguing that while all four companies are health insurers the Anthem deal is focused on commercial health insurance while Aetna's merger challenge is focused on Medicare Advantage.

"There are no material efficiencies to be gained - and likely only confusion - from conducting the proceedings jointly," Anthem said in its filing on Tuesday.

The company also asked the court to rule on its merger by the end of the year in order to have enough time to win approval from state insurance regulators, several of which suspended their reviews.

"Anthem believes that the merger must close by April 30, 2017 or Cigna will declare that it is terminating the merger agreement the next day," Anthem said in its filing.

Deciding the case will be Judge John Bates of the U.S. District Court for the District of Columbia. Bates was named to the bench in 2001 by President George W. Bush, and handed the government a big defeat in a 2004 merger challenge when he allowed Arch Coal to buy Triton Coal Co.

"He is a very favorable draw for the defendants in this case," said Stephen Weissman, who was on the team that won the Arch Coal case. "He's not antagonistic to the government, he's not. But he scrutinizes. He doesn't defer."

Anthem has argued that its planned $45-billion purchase of Cigna will give it greater leverage to negotiate better prices from healthcare providers and pass those savings to consumers, including those signing up for "Obamacare" plans on public exchanges.

The Justice Department, for its part, said the multibillion-dollar deals would reduce competition, raise prices for consumers and stifle innovation if the number of large, national insurers fell from five to three.

(Reporting by Diane Bartz; Editing by Bernard Orr)