Nintendo and Game Freak have announced the distribution of the Mewtwo Mega Stones for Pokémon Sun and Moon. They are planning on releasing most Mega Stones via distribution over a period of time. Beedrill will be the next distribution as it will be receiving Beedrillite and Mawilite for entering in the 2017 International Challenge which begins on March 7th 2017. A shared serial code will be given out in April for both of these.
This special event will start on March 7th, no word on when it will end – so be sure to do it as quickly as possible. In order to get the Mewtwonite X and Mewtwonite Y you will have to go to the Mystery Gift screen and type this code with the Receive code option: M2DESCENT