By Tim Reid and Richard Cowan PHOENIX (Reuters) -Arizona and Nevada election workers were toiling on Friday to tally hundreds of thousands of ballots that could determine control of the U.S. Senate and the shape of President Joe Biden’s next two years in office, in a vote count that officials in the two battleground states warn could drag on for days. Winning both contests would give either Democrats or Republicans a Senate majority, while a split would transform a Dec. 6 runoff Senate election in Georgia into a proxy battle for the chamber. In the Nevada governor’s race, Republican Joe Lombar…