Slide diatonic+tremolo (or octave). Has it been done?

[IaNerd]

What about this: a slide harp with some standard tuning (e.g. Solo, PowerChromatic) in one layer, but then pushing the slider opens up both layers, with the second layer being tremolo or octave of the first layer?

[EdvinW]

Brendan had something like this a few years ago, it might have been called OctaHarp Selectable(?) or something similar. (edit: Yep! Found it: https://www.brendan-power.com/harmonica ... atonic.php) It consisted of two combs arranged "in series", one behind the other, in such a way that the chambers would line up. The two combs had reeds one octave apart. The "selectable" part was that the back comb could be shifted sideways an thus be turned on or off. I played it once, and it worked well, though the mechanism wasn't as smooth as pushing a button.

The slide version of this idea, that you mention, sits high on my list of not-yet-implemented-and-might-not-be-in-a-long-while ideas, but usually to ad harmonics other than octaves. Fiddlers often add in open strings next to the one they play the melody on, and I've thought this could be nice way to emulate that and make the fiddle tunes a bit more authentic.