MAINPURI, April 20 -- Bahujan Samaj Party (BSP) chief Mayawati and Samajwadi Party (SP) patriarch Mulayam Singh Yadav - bitter political rivals for over two decades - sat together on stage in Mainpuri on Friday and asked workers of both parties to ensure their once-inconceivable alliance defeats the Bharatiya Janata Party (BJP) in Uttar Pradesh and topples Prime Minister Narendra Modi at the Centre....