Blythe, 21, made 19 appearances for Leigh on a similar deal last season and has recently signed a new deal with the Challenge Cup holders, while Armstrong was a regular in the St Helens Under 20 side in 2010, scoring 24 tries in 17 games.
Centurions boss Ian Millward said: “Blythe rejoins us from Warrington. He’s very well liked by all the players here. That was one of the reasons why we were confident of getting him on board and he also played in over threequarters of our league games last year.
“Tom Armstrong is a tall centre at 6’3 and scored a try against Hull KR for Saints at Edinburgh during the Magic Weekend. We pinpointed both of these players because we believe they can give us that extra thrust out on the edge in the outside backs.”
The dual registration system came in to force at the beginning of 2010, and Leigh were one of the champions of the idea, benefiting hugely from an agreement with Warrington which saw Blythe, Tyrone McCarthy and Lee Mitchell feature regularly for the Centurions last season.
Armstrong follows in the footsteps of former team-mate Jacob Emmitt, who has now left Saints to join Castleford, and spent time at Leigh last year.