Multiple mouth organs on a spindle have been around for a long time - Eddie Wallace from Glasgow uses one with all 12 keys (weighs a ton). 6 is the commonest; you can buy the spindles from Hohner (though my guru Iain Grant latterly preferred to put Tombo moothies in his, as they were better and cheaper). They are usually laid out in fifths; Iain's were Bb, F, C, G, D, A. Do the Bluebell Polka in both F and G and you can get through them all.

If anybody ever found a reason to use 2 semitonally separated moothies the same way as a chromatic, it will probably have been Sam Hinton. Try his set of instructional moothie videos on YouTube (hosted by the New Age pianist George Winston).