You may remember a piece played by Charles Hughes (ChuckH) called A Trumpeter's Lullaby, written by American composer Leroy Anderson in 1949.
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https://app.box.com/s/uiov301m57msxn4xtxkmqnjbasu0is7z
Listening to that, I made this comment to Charles:
Loved that combination of Trumpet and Horn. Tell me, as we can hear Trumpet (on the Upper?) and Horn and chord harmonies (on the Lower?) I presume that you do not actually have three hands, but did a 'double recording'? Multi-tracked it is the expression, I think.
Charles' reply was as follows:
Hugh .. I split the lower keyboard and put a horn on the right of the split which I occasionally brought into play by stretching my right hand between upper and lower keyboard; I've not tried doing a double recording as yet. Stretching to play two keyboards with one hand does take a lot of slow practice which, in the early days, seems to be getting nowhere, then one day it just sort of comes together. I had the solo Dixie Trumpet on the upper keyboard. I used the right foot switch to change the registrations.
... so no "Multitracking" going on here.