Mixed Time Signatures

I would be extremely grateful if you could tell me how to have mixed time signatures across different instruments within the same bars of music. i.e. Cornets playing in 12/8 whilst the bass end of the band are playing in 4/4.
I have played / conducted music where this has been done: Jesu Joy of Man Desire - which was written in 9/8 (for those with quavers that would have been triplets in 3/4) but 3/4 for others.