Is it okay to say "Now the light is shining twice brightly"? While Google Word gives multiple results for "twice brightly" (there is even a novel with such name, it surprisingly gives only one result for "shining twice brightly", which makes me have some doubts.
"Twice as brightly." "Shining twice brightly" implies that the light is shining twice, which doesn't really make sense. "Twice as brightly" implies something along the lines of "The light is shining twice as brightly as a normal light." (If you want more information on using "as" for comparisons I would suggest a Google search for "similes".)
And although not technically correct, most native speakers would just say "twice as bright" rather than "brightly".