I have met quite a few of these athletes. Martial artists, dancers, cheerleader, pole dancer and even yogis.
So what would hold a middle splitter back from a True Front Split? (Hips Squared). As you already know (or will find out right now) the side (straddle) splitters just need flexible inner thigh muscles, plus one hip flexor (pectineus) and inner hamstrings to a moderate degree.