Johnson, 26, was a fourth-round draft pick of the 49ers in 2014. He spent his first four seasons with San Francisco, playing in 63 games with 22 starts, while making two interceptions, 21 pass breakups and 145 tackles.
Johnson started all 16 games last season.
Logan, 28, was a third-round pick of the Eagles in 2013. He spent four seasons in Philadelphia and last season in Kansas City.
He has played 74 games with 63 starts, making seven sacks and 219 tackles.
The Seahawks are looking for defensive help after several offseason changes, including the departures of Richard Sherman and Michael Bennett.