By Trevor Hunnicutt and Steve Holland WASHINGTON (Reuters) -President Joe Biden met on Friday with families of two Americans being held by Russia, and personally reassured them he is working to gain freedom for the detainees. Biden sat down in the Oval Office with Cherelle Griner, wife of women’s basketball star Brittney Griner, and Elizabeth Whelan, sister of former U.S. Marine Paul Whelan. Biden conducted the separate meetings to assure the families that the detainees “are at front of mind,” said White House press secretary Karine Jean-Pierre. “We’re going to continue to do everything that w…