Fermata in four voice pieces: Inner ones visible in single-staff but not visible in combined staffs

[Nils Gey]

Hey people,

I have vocal pieces which have all fermatas in all voices so that I can print 
each voice alone. But for a reduction to a 2-staff system I only want the 
Soprano and Bass fermata visible (top first and bottom of second staff) but not 
the inner ones.
Do you know a way to do that? I currently use the voice presets \voiceOne and 
\voiceTwo.

Nils