Chapitre 1: Avatar theory behind multiple elemental control

I believe that anyone can use more than one element but the body won't be able to handle the energy clash between two elements let alone four; In the case of the avatar, Raava stabilizes the elements and keeps them from clashing. But it was said in the series that Raava keeps the elements stable until the avatar masters the four elements, which probably means that once the avatar masters the elements they can stabilize it on their own and they won't need Raava to help them unless they want to enter the avatar state which depends entirely on Raava (Raava can do more than that but I'm not gonna list everything). So theoretically having something to stabilize the elements will allow people to use more than one elements. Another method is mastering one element then closing the chakra point and opening another element chakra point, once the person masters all elements using this method they can open all chakra points to use the elements simultaneously.