Selective partCombine

[David Sumbler]

The only orchestral piece I have set in Lilypond until now was for 12
strings and a couple of other instruments.  I had 2 versions of the
strings material, one of which kept all of the parts on separate staves
for the parts, and the other which combined them on to fewer staves
(where appropriate) for the score.  This worked tolerably well: I wrote
one version first, then copied and edited it to produce the other.  In
simpler sections I could even make the necessary changes by using tags.

But I feel that there must be a better way.  As I am now about to start
on a work for full orchestra, I have been experimenting with having
separate parts which can nonetheless be combined on to a single stave
using \partCombine and its cousins. For instance, I have: 

instOne = {c'4 d' e' f' | g' a' b' c'' | d'' e'' f'' g''}
instTwo = {a4 b c' d' | e' f' g' a' | b' c'' d'' e''}
instScore = ?

The form of instOne and instTwo would be correct for the parts.  I am
looking for a way of writing instScore so that it combines the parts on
to one staff in bar 2, while keeping them separate in bars 1 and 3. 
(Obviously this would be senseless here, but imagine that each bar
represents a passage taking up a full page width or more.)

Is this possible?  Or am I approaching this in completely the wrong
way?

David