I am experimenting with this idea since several years. Since its arrival in 2016 I play the Seydel Sampler. This is a double-diatonic with a slider to interchange between 2 keys. It is substantially the same as Brendan's discontinued Irish Session Harp (ISH). Meanwhile I play the Seydel Sampler and the ISH in several default key combinations.
The 2 keys are just layouted one fourth / fifth apart. This allows to change between two diatonic keys without changing the harmonica, only by a push of the slider. Usually this is thought to change the key between the tunes of a set or between two parts of a tune (e.g. AABBAACA or alike). Therefore Seydel is furnishing the Sampler with a slide lock to avoid unintended change of keys while playing a tune. Brendan omitted the slide spring completely on the ISH for the same reason.
But since I began using the slide frequently while playing the tune to get nice forth/fifths jumps and trills and alike, I removed the slide lock on all of my Samplers and attached a snare to the slider of the ISH, where I could hook in a finger to move it out suddenly when I want to pull it out and push it in and out ... frequently.
I found out that this is fitting especially to baroque and renaissance style music. Here is a little improvised tune with my Sampler D and A mixolydian. Both of those keys contain exactly the same notes. (I am sorry for the poor sound quality. It is recorded live with a smartphone out of the audience from quite a far distance, in a festival tent where there was much talking and laughing among the audience.)
Triona - Fantasie Barock Teil 2
Klingenthal, Mundharmonika Live, 22.09.2019
https://soundcloud.com/triona-966519605 ... rock-teil2
And the next step following this idea was leading to a entirely different outcome. I took a Sampler in standard tuning D+A and removed the slider entirely and replaced it by a normal mouthpiece of a chromatic. This allows to play straight power chords provided by the fifths/forths i each hole. Brendan has heard me playing this monster of volume in Klingenthal. Maybe he can remember. I played it with the band - without any amplification. If you play it with broad tongue block (octave split or more) you hear the power chord sounding by 4 reeds or more. If I play it for more than half an hour my head is ringing and spinning.

The downside: This setup allows only chordal playing. Single notes can not be played. I tried to heal this by using a nonslider mouthpiece instead of the standard chromatic mouthpiece. I intended to get the choice either to play a single note by using only one hole or to play the power chord by using the 2 holes above each other in order to change fluently between power chords and solo playing like on a guitar.
But this did not work as I had intended. It allows to play single notes indeed. But the split up air stream through 2 of the holes of the nonslider mouthpiece is far to weak to bring the power chord to full sound like it does with the chromatic mouthpiece. So I put this project on hiatus.
dear greetings
triona