Complex sentence: He visits Sakharine in Marlinspike Hall and accuses him of the theft when he sees a miniature model of the "Unicorn", but when he notices that Sakharine's model is not broken, he realizes that there are two "Unicorn" models.
Split into simpler sentences: He visits Sakharine in Marlinspike Hall and accuses him of the theft. When he sees a miniature model of the "Unicorn", he notices that Sakharine's model is not broken, realizing that there are two "Unicorn" models.