IMHO: If you are in a situation where you need both, you have problems!

Herbicides need to go out early, when weeds are small, easy to control, and most likely to interfere with tillering. If you have a disease problem then, too, you may as well let the weeds have the crop! The best fungicides only give about three weeks protection.

If you have weed problems at fungicide time, chances are the weeds have already adversely affected yields. Herbicides at this time would only fit a "salvage" type application.

DISCLAIMER: While these comments are intended to be informative based on real-world experience, what happens your production area in your field may be so different that these comments do not apply