Several German pipe organs built in the 19th century by the Walcker firm, of Ludwigsburg, such as the 1845 instrument (destroyed in 1944 and rebuilt twice since) for the Stiftskirche, Stuttgart (photo), were supplied with two pedal keyboards, or claviers, one placed above the other.
The primary impetus giving rise to this curiosity in organ building, a Walcker specialty, was that Germany was very slow in the 19th century to adopt the concept of the swell in the organ, and dynamic variety was created by changing claviers, with the louder, bolder sounds assigned typically to the lowest clavier, and the softer sounds assigned to the highest one.
One of the ways a decrescendo was secured was by passing from the lowest to the highest claviers in succession.
The second pedal clavier, or so-called "upper pedal," served two basic functions: 1) to provide soft bass lines in contrast to the loud main pedal, something that comes in handy when there are quick alternations between loud and soft passages, and 2) to allow the feet to play slower solo lines, either at low or high pitches.
This upper pedal operated in the plane that most of us use for the Swell shoes, so it would feel as comfortable to play as our current Swell shoe assemblies, although, obviously, it would take some time to get used to it.