How to make beams non-slanted

[Sven Axelsson]

I am creating a context for pipe band snare drum that builds on
RhythmicStaff, but allows for two pitches to indicate left or
right hand. This uses note heads on either side of the single
staff line. I think that it would work better if I could get all
beams aligned and non-slanted, i.e. {left8[ right8]} should produce 
a horizontal, unslanted beam.

I first thought that this could be done by tweaking the Stem
properties, e.g. beamed-lengths, beamed-minimum-free-lengths and
so on. But perhaps it can't be done this way. Can anyone give me
some tips on how this could be done?

-- 
Sven Axelsson