Watkins is a midfielder and arrives on loan from SkyBet League Two neighbours Cambridge United.
He featured a number of times for the Uís during the last pre-season and signed his first professional contract at the club ahead of the summer.
He is recently back from a second loan spell with Bostik League North side Mildenhall Town, for whom he has featured in 20 matches this season.
Shackleton is a striker and joins on `work experience` from League One Peterborough United.
He has had previous loan spells with United Counties League Premier Division side Pinchbeck United and, most recently, Evo-Stik Northern Premier League Division One East outfit Wisbech Town.