Sarsfield, 21, is a former Wigan junior and is a free agent after spending time out of the game after leaving the Giants. He had a trial with Widnes in the close season.
Sarsfield signed a three year deal after leaving Wigan but was released a year early.
Harvey, 24, came through the Warrington system and played five games on-loan for Swinton in 2010.
He was sent on loan to Wakefield but decided to quit the game and return to work as an IT technician with a leading oil company.
Swinton head coach Gary Chambers said: “Matt is a quality player who has grown up in a top environment at Wigan and Huddersfield and is a fantastic addition to our squad.
“I’ve known Craig a long time and he’s a big and clever stand-off. He only left the full-time game because he had a really good job to go to.
“We are trying to build something for the future here at Swinton and these two young, talented lads fit the bill.”