i have the same problem with my knees, there isn't anything you can do to fix it but if you want to do oversplits, have someone either hold your knee from extending or put like a foam block under her knee to keep it from bending back all the way.
I would be careful with pushing down on your knees in the split. I can do right leg oversplits down on over 2 feet of mats. So anyway, I was just doing regular splits on floor and I noticed there was a little light coming through under my knee. Since I usually did oversplits, I hadn't noticed this. I pushed down on my knee, and it ended up feeling bad/ being touchy/not reliable for about 3 weeks. So, I would really not recommend doing that. I personally like stretching splits with whole body on top of an old resi (you really squish in) or stretching splits on your back with a theraband (or partner) That way puts less pressure on your knees than stacking mats.
Way back when I had all three splits and a bit of an over split on my right, but nothing to crow about. Flexibility wasn't my thing and had to work hard for that middle. Good thing is I didn't loose much in my older age. Still have that right, can push down that left if necessary, which isn't often unless I challenge a student, the middle is good for an old lady but not down anymore. Better than the average Jane off the street.
For those natural Gumby knees (everyone knows who Gumby is right?), I tell my gymnasts with this luck/affliction to "choke" up on the stack mat. Put the whole lower leg, calf all the way to under the knee on the mat. Big thing here is to then put your hands on the mat too and make sure your chest is up so you can really stretch that hip flexor. Putting your hands on the floor will make you lean too much over your front leg, while you might get a good hamstring stretch there, it isn't very effective if you want a leap that keeps your chest up or lay back into a ring leap/jump. Hopefully we all want a leap with our chest up right?
i used to have all 3 splits down when i was like 6, but it didn't stay that way. A few years ago, i really wanted my splits down, so i slept in my right leg split. yes, all night long. When i got up the next morning, i could hardly walk, but my split was down and has been down ever since. I just won't be doing that again any time soon!!!