The pair have been training with the Hull first team in pre-season, and now go straight into contention for York’s Northern Rail Cup opener against Swinton on Sunday.
Green scored nine tries in five games during a similar spell at Doncaster last season.
Hull FC director of rugby Shaun McRae said: “We are delighted that Chris and Laurence have been dual registered with York and we feel they will really benefit from playing a higher standard of rugby with the Knights.
“York have recruited exceptionally well for the new season and boast a squad of undoubted talent. Working with coach Chris Thorman and the experienced players in the York team, many of whom will be known to Hull fans, will really help their development.
“It will be particularly beneficial for Laurence who is still learning the game, but will now have a chance to test his natural strength, size and skill in a tougher competition.
“As for Chris, he has been a consistent performer for the Under-20s and impressed in the pre-season friendlies, so he will be using his opportunity at York to push for a place in the first team.”