(Reuters) - CBS Corp and Viacom Inc have reached an agreement in principle, sources familiar with the matter told Reuters on Tuesday, taking one step closer to reuniting media mogul Sumner Redstone's U.S. entertainment empire after 13 years apart.Viacom shareholders will receive 0.59625 CBS shares for each share they own, representing a slight premium to Viacom's closing price on Monday.Shares of Viacom rose 2.3% to $29.19 and CBS shares rose 2.4% to $49.20 in morning trading.The two companies..