Lilypond: same voice

[Helge Kruse]

How should I implement this in Lilypond?

The excerpt shows a piece for one or two harps. If played with one harp 
lower staff is for left hand upper staff for right hand.
When played with two harps each staff is for one harp.

In the first measures both instruments should play unisono. How can I 
write theses unisono tremolo triplets? Should I write it two times with
   \change Staff
or is there a more elegant way to write the same melody only once?


Helge

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