The Whites sent a scout to watch Peterborough's Boxing Day clash with Bradford City where Maddison set up both of Marriott's goals in a 3-1 win.
Marriott, who also set up his side's opener, now has 21 goals for the season - including 17 in League One.
The 23-year-old from Beverley was linked to Queens Park Rangers earlier this week while both Aston Villa and Sunderland are understood to be interested in signing him.
Maddison, 24, has been in incredible form this season, setting up 23 goals for Posh and scoring five himself.
Leeds were in talks to sign the Durham-born attacker earlier this year but negotiations between the clubs broke down amid reports of former Whites owner Massimo Cellino 'meddling'.
Speaking in March, Peterborough director of football Barry Fry said that Maddison was "gutted" that the deal floundered.