Paul Singer has landed in a once-in-a-cycle opportunity. When his hedge fund, Elliott Management, and private equity firm Vista Equity Partners agreed to acquire technology company Citrix Systems (CTXS) for $16.5 billion in January, leverage came easily and cheaply. Market conditions then turned, leaving the debt backing the Citrix deal languishing on banks' balance sheets. Now, Elliott has bought a chunk of that debt at a knock-down price, potentially benefiting from the banks' pain twice over.