I love the double course instruments of the folk music tradition. I have experimented with making mandolas with different types of bridges to see how it would affect the sound. I feel that the regular floating bridge alternative gives some bonuses in volume and a strong mid to punch through an ensemble, whereas the glued bridge gives a more pleasant ”musical” tone, rich in overtones, but on the cost of volume.
I have build with a 17 inch scale and even with a shorter 15,75 inch scale for a 5-course version including the high E strings of the mandolin.
I would enjoy experimenting more with multi course instruments, maybe fan-frets or other features could be incorporated.
There are as well pictures here of a charango and a tiple i built.