Pre-emergent herbicides generally interfere with the germination process of seeds. They will not kill perennials or plants which are already growing. With several weeks gone by, they won't interfere with the activity of a post-emergent.

So, go ahead and use the post-emergent.

However, be sure to first identify the weeds you are trying to kill so that you will be sure to use an effective herbicide.

If the "weeds are a bloomin," it's too late to use herbicide. AT the very least, remove the flower heads so that they don't seed about.