Customers cash in Slurpee rewards points for concert tickets, try out flavor mixing with 7-Eleven’s new dual-chambered cup. 7-Eleven Inc. is mixing up the 2011 Summer of Slurpee with a mash-up of cool cups, cold concoctions and sold-out concerts. Not only is 7-Eleven encouraging Slurpee flavor-mixing with its MixMaker dual-chambered cup and valve straw, the…