This is a routine that was done by a pair of twins that I came across at a juggling convention many years ago. I’ve forgotten their names but to this day they had the best side splits I’ve ever seen, which was made even crazier by their complete lack of flexibility in other positions.
Seems to draw influence from chinese kung fu/wushu, so uses a lot of pulsing to develop range. My only prerequisite is that you can sit on the floor in a pancake.
1 – 5 reps each side, alternating sides
2 – 5 reps hinging forward while trying to open out the hips forward by engaging the glutes
3- Then lift your hips and widen your legs
When talking to the twins, they recommended between 200 – 400 reps a day in each direction, with a noticeable difference between the two, as one did 200 and the other 400. Start with 50 – 100 reps daily in each direction to start, as 1 continuous set and progress from there.