The Saints pair had talks with the Hillsborough club this morning and completed their deals with hours to spare before the end of the transfer deadline.
19-year-old Best started two games at the beginning of the season and scored the club's opening goal of the campaign against Hull but then broke his foot in the same match.
The striker has been linked with a move to Crystal Palace in recent weeks but it looks as if he will move north again to provide cover for Drew Talbot and competition for Steve MacLean, Deon Burton and Marcus Tudgay.
Togo-born Folly was signed as a 17-year-old by Southampton and played in the Premiership for them - three years on he has had loan spells at Preston and Nottingham Forest and regarded as one for the future at St Mary's.
The France Under-21 defensive midfielder will be looking to be an ample replacement for Steve Adams and Craig Rocastle, who are both out injured.
Best will wear Lee Peacock's old number 10 shirt while Folly will have Graeme Lee's old number 4.