The 737 800 and 900 has three temperature zones (I think the 400 is the same, but I've never flown it), but like the other models, has only two packs. So, the packs produce air conditioned air at the coldest of the temperatures selected in the three zones. The warm trim air is then added to the ducts to the two areas with higher temperature demands.
It is not fitted to the 600 or 700, which are also NG models.