Printing two staves in the same place (overlaying)

[Daniel Benjamin Miller]

This is going to sound unusual, but is there any way in which one can 
truly overlay staffs? I mean engraving a staff directly on top of 
another, not taking any collisions into account, etc. (I know how to 
combine voices; that's not what I'm asking about.) Is there any way to 
do this via a hack (of spacing or the like)? Ideally, the second staff 
is manually set to align the exact same way as the first, so they really 
do always coincide.

All the best,
Daniel