Billionaire investor Steve Cohen. Steve Marcus/Reuters Steve Cohen's Point72 and Ken Griffin's Citadel are investing $2.75 billion in Melvin Capital.Melvin is down about 30% this year as its short positions are getting hammered.Day traders have bid up the stock prices of GameStop, Bed Bath & Beyond, and other popular shorts.Visit Business Insider's homepage for more stories.A pair of billionaire investors are swooping in to support a short-selling hedge fund in its battle against an army of irreverent day traders.Steve Cohen's Point 72, Ken Griffin's Citadel, and other partners are plowing a total of $2.75 billion into Melvin Capital, the hedge funds said on Monday.