The 17-year-old’s, who play for Pogon Szczecin, will travel to the Championship club’s Thorp Arch HQ next week.
Boniecki is an attacking midfielder, who has won five caps for Poland at Under-18 level.
Central defender Sadowski represented Poland once at Under-15 level.
Dariusz Adamczuk, the president of Pogon’s Academy, said: “For both players it is a new experience and an opportunity to test themselves against the background of players from England.”
Neither player has made a first-team appearance for Pogon, who compete in Poland’s top division, the Ekstraklasa.
Leeds chiefs are strengthening their Under-23 squad as part of a long-term approach to the transfer market.
Goalkeeper Kamil Miazek, 23, put pen to paper on a short-term deal while forward Oliver Sarkic, 20, turned his loan move from Benfica into a permanent deal earlier this month.
Promising striker Ryan Edmondson, 16, was snapped up from York City on a long-term contract in November.
Leeds are also hoping to beat Everton and Brighton to Forest Green Rovers wonderkid Jordan Stevens.