Countless fans have considered themselves to be from more than one Hogwarts House, and now J.K. Rowling essentially confirmed that the House hybrid does exist, though is rather rare.
While obviously no one can be Sorted into two Houses at Hogwarts, those who have strong characteristics of two Houses - like Harry Potter, Hermione Granger, Severus Snape and Neville Longbottom, for instance - will probably get the chance to choose their House after some deliberation, or the Hat may choose the 'right' one for them after a lengthy Hatstall.
Those who have felt they belonged to more than one House can now be validated as J.K. Rowling said they do exist.