Keinhorst, a Germany international who played for Leeds in the 2012 Challenge Cup final at Wembley, boasts 15 Super League appearances but was down the Rhinos pecking order for 2014 while Briscoe, the younger brother of England winger Tom, is set to make his senior debut for the Wildcats.
They become the seventh and eighth new faces for Wakefield ahead of the new campaign and will join former Leeds team-mates Paul McShane and Richard Moore at the Rapid Solicitors Stadium.
"We felt as though we need some competition for places and Leeds have got an embarrassment of riches in the backs which gives Jimmy the chance to come and play first grade and get some experience," Wakefield coach Richard Agar said.
"He's had some good experience so far and played in some big games and whatever level he's played at, he looks like a player who can get you a try.
"Luke is a strong, natural footballer who can play anywhere in the backline and can slot into the back-row if required. He'll add depth to the squad and I anticipate him playing first grade this season."
Leeds coach Brian McDermott said: "Jimmy and Luke are exciting prospects for the future and the chance to play in Super League this season is a great opportunity for both of them."