This may require some effort with the "punch punch kick" combo, but it may be easier with a rifle or SMG. When an enemy is prone on the ground in the hold up posture, move toward the torso. After the kick or swing of your weapon, immediately press X to crouch. If done correctly, both characters will flinch and the enemy will be now be standing in the hold up posture. This is much easier than knocking them out to shake them or then waking them up just to hold them up again.