We can't really tell if who owns the second biggest bitcoin wallet because there is a lot of big people to choose from but if i am allowed to pick for a guess then i will go for Bill Gates, he is the richest man and he already have a video for about a few years ago that he said that bitcoin is unstoppable and i believe that he invested in bitcoin and he is still holding bitcoin for long term and he owns the second biggest wallet because of his capability from his wealth.