By Prashant Mehra MUMBAI (Reuters) – Conglomerate Reliance Industries Ltd and top mobile operator Bharti Airtel Ltd have agreed to share network infrastructure, as the old adversaries in Indias crowded telecoms industry set aside differences to save costs. The companies, headed by longtime rival billionaires, undercut each other during Reliances last foray into telecoms nearly a decade ago. Reliances Mukesh Ambani, Indias richest person according to Forbes magazine, and Bhartis Sunil Mittal, Indias 10th richest, will continue to be rivals in the fast-growing retail market.