Narsingh, 21, and Assaidi, 23, both travelled to London on Monday to tour Fulham's training facilities and ground.
They were accompanied by their agent Mino Raiola, who began talks with the club over personal terms as Fulham look to strike a double deal.
Heerenveen confirmed they gave both players permission to travel to England.
"We have given Mino Raiola and the players the space to talk," Heerenveen technical director Johan Hansma told De Telegraaf.
"Narsingh and Assaidi had to have a good feeling with a club. It doesn't make any sense if we reach an agreement with a club and the players won't want to go there. To succeed in a transfer there are three sides needed."