Change of Time Signature for part of band only

Hi, can anyone advise me how to change the time signature for one half of the band score only.
I'm trying to write out a full score for Dances with Wolves but can't work out how to change the top half of the band only as the score has 4/4 playing against 12/8 at the same time. I can't find how to change the time signature on selected instruments only - it seems to be all or none. Thanks