The phrase "be double that of" is grammatically correct and can be used in written English.
Example: The cost of living in the city is reported to be double that of the rural areas.
Within six years, China's economy will be double that of Germany's, now the world's third largest.
Litvinenko was a fighter: the dose he received was estimated to be double that of those at the epicentre of Chernobyl, who all died within two weeks.
It is ridiculous that a male director's bonus could be double that of his female colleague, no-one performs that much better than their deskmate.
He would not disclose specific sales, but he said that after growing about 50percentt annually over the past six years, sales this year would probably be double that of last year.
Election workers at this suburban Columbia polling place, and at virtually every other around the state, said they had never seen such a heavy turnout for a Republican presidential primary, which appeared to be double that of four years ago.
Other studies predict that demand in 2045 will be double that of the present day [22].
