First of all, your registration should not be necessarily 8' based. If the piece has a playful character, I think you will find that 4' flute on one or two different manuals will work really well.
Another option would be to play the bass at the 16' level. You could simply choose 16' and 8' flutes but there are more colorful choices, as well.
This could be a very elegant and exotic registration, for example, Fagott 16' in the left hand and Vox Humana 8' (or Cornet, or something similar) in the right hand.
Bach contemporary Georg Friedrich Kauffmann (1679-1735) liked these strange sounds and left many registration indications in his wonderful collection of chorale preludes "Harmonische Seelenlust".
Try these registration tips in your organ practice. Such stop combinations will surely make your duos much more colorful.
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