The Mandolin Banjo is a crossover. In the heyday of Mandolin Orchestras and Banjo Bands, all sorts of instruments were produced. Banjos are quite loud compared with Mandolins, and Banjolins fall in between.
Stringing a Banjolin with double strings made it much louder - but it also became a different instrument - the Mandolin Banjo.
With banjos and banjo bands becoming popular and banjos becoming commercially available, mandolin players whose popularity was waning would have approached banjo manufacturers to see if a 'banjo-like-a-mandolin' could be produced. The result was more popular than either imagined.