Assuming you’re in NTSC-land:
1- work in a 720×540 square pixel comp (use the preset)
2- use Composition>Make Movie. Set the Output Module (OM) to stretch your comp to D1. If you want to render to DV, stretch to D1 and crop pixels 2 off top, 4 off bottom to make 480. This is your TV render. Make sure you are not rendering fields.
3- use Composition>add Output Module. Set this OM to stetch to 320×240 or whatever.
This way you are rendering one movie and scaling it down for two outputs. Faster than rendering two movies.
If you really want to render fields (I rarely do), you have to drag the 720×540 comp into a D1 or DV comp, and scale it down before rendering. Do not scale in the OM. Do the same by dragging it into a 320×240 comp … but never render fields for the web.
Does that help?