The phrase "twice as effective" is correct and can be used in written English.
You can use this phrase when referring to something that has been improved in terms of efficiency or effectiveness. For example, "The new program was twice as effective as the old one in improving student outcomes."
Drug-coated stents are twice as effective as bare metal devices in preventing reblockages.
To judge from the Google numbers, a Chicago-to-Honolulu move would be at least twice as effective as medication for your cold-months mood.
Even if by some miracle we had the precise number of missiles they have, they would still be twice as powerful, because their missiles are twice as effective as those we are currently planning to build.
With a recall index of 228, the commercial has proved more than twice as effective as the average commercial over the last five months, based on 100,000 respondents from across the country.
Blue sedatives are more than twice as effective as pink ones (although not among Italian men – one explanation being that they associate blue with the national football team, which gets them excited rather than sleepy).
The $400 Lumix DMC-FZ35, the $280 Lumix DMC-ZR1, and the $300 Lumix DMC-FP8 all offer 12-megapixel resolution and new Power OIS optical image stabilization that Panasonic says is twice as effective as existing Panasonic technology.
In a 2005 study in The Journal of the American Medical Association, Dr. Shear presented evidence that the treatment was twice as effective as the traditional interpersonal therapy used to treat depression or bereavement, and that it worked faster.
The new speed dependent technique considered here is theoretically twice as effective as traditional techniques.
A soluble π-conjugated polymer cis-PmPV is found to be twice as effective as its trans-PmPV isomer in dispersing SWNTs into organic solvents.
A survey by Neumark (2013) of US evidence highlights that, using unfavorable estimates, hiring subsidies are at least more than twice as effective as public job creation.
Furthermore, interventions conducted in the participants' native language (if other than English) were twice as effective as interventions conducted in English (Griner and Smith 2006).
