Polychoral?

I'm writing an oratorio, for the following forces:

String quintet

Brass quintet (bass and tenor trombone, horn, two trumpets)

SATB choir (12 singers)

Timpani

In the final movement, they will separate into two choirs and sing antiphonally, in homage to the Venetian School composers (e.g. Willaert, Gabrieli).

You can listen to a good Gabrieli piece that uses this technique here:

YouTube - Canzon Septimi Toni - No. 2

My question is, what forces should each choir contain? What kind of setup do you think would be best and most balanced?

The most simple way to do choir I and II is just divide SATB 6 voices and 6 voices. I think that would be the easiest way and make a better performance. You could also divide by sex. like SSAA for I and TTBB for II. Any other way I think would be to complicated. But of course its up to you :)

Good luck.

If I split up the choir by gender:

CHOIR I: 3 Sop, 3 Alt

CHOIR II: 3 Ten, 3 Bass

How should I apportion the instruments?

Not exactly sure you'd have to apportion the instruments any different. Lower strings and 1/2 brass with the male chorus and then upper strings and 1/2 brass with female chorus would be a good appropriation. You could always just use the strings to double the voices and the brass/timpani the remainder of your music, etc... thats what a lot of composers in the past have done to good effect (or use some of the brass to double the voices as well?)

I mean, what effect do you want? You can do high/low singers and high/low instruments, you could do high/low singers and low/high instruments, you could do all voices in both choirs and high/low instruments... Depends on the effect. Give us a little more.