Winger Gwion Edwards, 19, joins from Swansea and joining him will be Newcastle starlet Mehdi Abeid.
Algerian Abeid - who joined Newcastle from Lens in 2011 - has not played in the Premier League yet but the 20-year-old has played in the Europa League for United.
Edwards is yet to make an appearance for the first team at Swansea, where he signed his first professional contract in July 2011.
A statement on the Swansea website read: "Young Swans winger Gwion Edwards has joined St Johnstone on loan in the Clydesdale Bank Premier League.
"Edwards, 19, will spend the rest of the season on loan at McDiarmid Park under the managerial guidance of Steve Lomas, who gave lengthy service to Man City and West Ham as a player, before coaching spells at Norwich and a return to the Hammers as a reserve team boss.
"The local Lampeter-product signed for the Swans' Under-15s squad before rising through the ranks to become an important part of the successful FAW Welsh Youth Cup campaign in 2010.
"He signed his first pro-contract the following summer and was set to make his first-team debut off the bench in added-on time in the final league game against Liverpool last season, but he didn't make it onto the pitch before the final whistle blew."