Hello there Michael,
This is a pretty interesting question you have here.I will give some stretching exercises you can apply but they'll be mostly for the upper part of your body.
The first one you can do is the lying abdominal stretch which you can lie on your front side and push upper torso upwards with your arms until you feel the stretch.
The second one is the lower back,that is,a mild back stretcg with your hands on the tiny part of your back mostly your waist,slightly bend back until the stretch is felt.Repeat 2–3 times.
The third one,for your upper back which is a latissimus dorsi stretch you begin by kneeling extending forward until you feel the stretch.Slide hands forward push your butts backward.Repeat twice.
As I already wrote elsewhere (Maurizio Puxeddu's answer to Is the ability to side split and front split due to bone structure?), basically everybody can perform sagittal splits (what you call “middle splits”). You just have to have enough patience and stretch every day for a month, a year or 10 years or whatever it takes.
About the naming of the splits: Maurizio Puxeddu's answer to What is the meaning of "splits" in ballet?
The answer below is about frontal splits (popularly called “side splits”):
This is a method I’ve been told by a coach but I never used or verified it or seen anybody actually using it, thus take it with a grain of salt.
You get down as far as you can in your frontal split (the technical name for what you call “side split”).
Someone else (typically your coach) gently presses your on your tight, near the hips, with their foot.
One of two things may happen
If you still go down just a little and/or feel pain in your muscles and tendons, you should keep working on your frontal split
If you reach a point where you don’t go down and you don’t feel pain in your muscles and tendons, you reached your limit and you won’t be able to perform frontal splits
I guess that if you take an x-ray of your pelvis and show it to a physiotherapist, they should be able to tell you whether you could do frontal split or not.
Do not try the method above at home with your friend. Even if coaches are not physiotherapists, they have some experience in manipulating the bodies of little athletes and are accustomed to do things like that without risking to break a bone. You are not necessarily qualified.
Thanks for the A2A, Johan. Would you please ask the same question about the frontal splits so that I can place the wrong answer in the correct question? That would be very nice of you.