The progression from two words to one word through a hyphenated stage is not otherwise unknown. The three forms base ball, base-ball and baseball coexisted all through the 20s and 30s before the single word form became the standard. The two word form was the standard through the 19th century. Otherwise there are pairs where the single word form is a noun and the double word form a verb although I have seen backup as a verb with backupped and even backuped(!) as past tense versions.