Train the one-two as a single movement: a first-time set, an early spin, a run timed to stay level with the line, and a finish across the keeper from the return.
Set three mannequins as a back line about 20 metres from goal, the middle one a metre nearer goal than the two outside ones, about 5 metres apart. Put a start cone about 31 metres out with a pile of balls a few metres behind it. Full goal and keeper. One player stands behind each outside mannequin and the rest line up at the cone. The first player in the line plays into the player behind the right mannequin, who comes short to meet it and sets it inside first time. The player behind the left mannequin checks into the middle, takes a touch across his body and slides it through the gap between the middle and right mannequins. The right player spins round the outside of his mannequin, stays level with it until the pass is played, runs onto the ball, takes a touch inside and finishes across the keeper. Rotate: the passer goes to the left mannequin, the left player goes to the right, and the shooter jogs back round the outside, collects a ball from the pile and joins the line.
A one-two is one movement, not two passes. The player who sets the ball is already running as it leaves his foot, and the player who returns it is passing to where the runner will be, not where he is. Most one-two drills break this into pieces: a touch to control, a look, a pass back to feet. This one is built so that the pieces do not work.
The right player comes short to meet the first pass and sets it inside first time. If he stops it, the spin is late and the gap has closed by the time he turns. The middle player gets one touch, across his body, which is exactly enough to open the gap between the mannequins and not enough to admire the picture. The return goes into the space beyond the line, round the outside of the right mannequin, and the runner has to arrive there at full speed.
The timing problem is the one that matters. The runner must stay level with the mannequins until the pass is played; going early would be offside in a game and here it leaves him waiting for the ball. Going late leaves the pass rolling into nobody. Players learn to read the middle player's touch as the trigger, which is how a one-two works against a real back line.
The finish is across the keeper from a ball arriving at an angle. A first touch inside and forward keeps the angle open; a touch away from goal closes it. The rotation sends every player through every role, so everyone sets, returns and finishes in each round.
Save it to your private drills, then change the animation, adjust positions, swap equipment, or drop it into a session block. Or start from a blank pitch and build your own.