Caldwell, 27, has started 19 games over the past four seasons for the Houston Texans after being drafted in the third round in 2009. At 6-foot-3, 315 pounds, he can play both guard and center, and could compete for the open left guard spot.
Hills, 28, was a fourth-round pick of the Steelers in 2008. His most extensive NFL action came last season with the Indianapolis Colts, when he played in six games, including his first career start. Hills can play both tackle and guard.
The two signings bring the roster to 88 players as the Bills are set to hold their first practice of training camp on Sunday evening.