Thomas spent last season at Leeds - the club Davies left for Llanelli - while Bater, 26, joins from the Ospreys.
Davies said: "Nathan is one of the most experienced and respected players in Europe and one of only eight Welshmen to have played in a Heineken Cup winning team.
"Having worked with him at Leeds I know all about his qualities and he will bring a hard, competitive edge with him.
"As for James, he is one of the brightest young back row talents in Wales.
"He has displayed good leadership qualities at his former clubs and deserves more than the one Welsh cap he has under his belt to date."
Thomas, 30, won the last of his nine caps against South Africa in 1998 and spent six seasons at Bath.
Bater, who made his solitary international appearance against Romania in 2003, has played 19 times in the Heineken Cup for Swansea and the Ospreys.