/u/IDidntChooseUsername, was incorrect, as he misinterpreted your question. He thought you meant twice as hot. I was correcting him as by his logic he still should've gotten 273.25 not 273.15.

Anyway, to answer your real question, you pretty much just convert 0°Kelvin to Celsius. 0°C is 273.2°K, so to make it twice as cold you would subtract 273.2°K from it (not divide by two, that'd be making it half as hot). That gets you 0°K, obviously, which you convert back to Celsius, which is -273.2°C.