The Sensex and Nifty gained for a second straight session on Thursday on short-covering in stocks such as Bharat Heavy Electricals, even as the overall sentiment stayed cautious due to uncertainties surrounding the ability of global central banks to stimulate growth.The Sensex and Nifty gained for a second straight session on Thursday on short-covering in stocks such as Bharat Heavy Electricals, even as the overall sentiment stayed cautious due to uncertainties surrounding the ability of global central banks to stimulate growth.