Once upon a time there was a girl named Melonie, who lived in the countryside and ran her own melon farm. The actual melons aside, her own melons were pretty big themselves and she was so popular than when she gave birth to adorable twin boys it was way difficult for anybody in particular to claim to be their father, so she named them Diego and Lucas and decided to raise them up by herself at the melon farm.
The boys were very nice and loved their mom very much, and like many young countryside kids they loved playing "bendy games" in the backyard of their house. Almost since day one Diego's favorite became bending backwards while for Lucas it was frontbending that he enjoyed the most. They looked the same, so the only way to tell them apart was to ask each boy to show off.
Their favorite game was to mirror each other's poses while bending in the different directions. Melonie had a huge mirror in the house, so in the evening the boys would spend a lot of time in front of it to polish the matching of their poses to perfection, while Melonie loved to watch them and telling them how good they looked or what they should change.
In their teens the boys became so enthusiastic that they decided to become professional contortionists and make a living off their shows. Their faces looked identical, but you could always find out who is who, simply by asking: "Can you show me that amazing backbend everybody's talking about!?" If the boy gets annoyed, that's Lucas, he can't bend backwards much further than most teens his age. But if he bends backwards, folds in half, passes his shoulders between his own thighs so they end up under his armpits, then lifts his head to his very crotch, grins at you and you just can't believe what you're seeing, then you surely have Diego. Or you can ask about a "frontcurl" and the reaction will be just the same but opposite XD
Thanks to TK for the idea of mirroring a backbend and a frontbend ^^