For a quartet of two high range and two low range instruments.
In the manner of organum, create smooth, sinuous, slow moving melodic lines that are coupled at the perfect fifth in like registers (i.e., the upper range melody is doubled a P5 lower; the lower voice is mirrored a P5 higher).
The degree of chromaticism is up to you. Let the lines freely mingle.
Extension: experiment with coupling different interval types. Perhaps you could begin with very small interval pairings and grow into very wide intervals.