How do you change a characters default movement?

The goal i have in mind is that the characters movement starting position is with his knees infront of him(The posture is like he is sitting in a chair) and when you press the action button (whatever that will be) the legs straighten out underneath him-a jumping movement.
as soon as you stop pressing jump the jumping animation reverses and his legs revert to being in that chair like position.The force of the jump will be determined by how late in the jumping animation the impact between the ground and the characters legs are(So if you have nearly a full extension of the legs you will jump with much more force.

The other dynamic in the movement is that the direction of the jump will be set on some relative angle of the legs on a pivot from the hip so if the upper leg is at X then the jump angle will be Y this ratio will likely be 90 degrees.

The final movement challenge is how do i make 2 players share one pawn ? The players will be given a leg each to jump with and control im not sure how to set up such a dynamic but is basically the final part of the game dynamics.As each side of the body will be weighted so if one side performs a jump it will set the body into rotation in the air unless the other side jumps with equal force.

I have scanned many youtube videos and real alot in the forums but i cant find how to set up such a dynamic.
Anyhelp would be greatly appreciated.