As usual, Hermione was much further along than she thought she was. Harry had plenty of time to shrink and pack her books into one box, and then cast a massively powerful weightless charm on it. Hermione had most of her other things in her bottomless bag, and it was only a matter of preparing a few more delicate items for magical movers, who would transport the unshrinkables via broom or Apparation.
By the time Harry's silent alarm for Ron's arrival went off, Hermione had integrated most of her furniture and knick-knacks seamlessly with Ron's. They had just finished turing one of the spare bedrooms into a deceptively large library office before they heard the alarm. As one, they ran out into the living room and flopped onto the sofa together, both of them pretending to read Quidditch magazines as Ron came in the door.