How do I get rid of dandelions and crab grass in my lawn?
If this is a homeowner doing his own lawn, there are combination herbicides that can work on both weeds. 2,4-D + dicamba + MCPP/MCPA are typical combos that get the broadleaved weeds such as dandelions. Quinclorac herbicide will control most crabgrasses but must be treated when real small or larger at 5-6 leaves stages. Somehow, it supposedly misses a mid growth stage probably when it starts tillering.
If the infestation is light, it may work with 1 app. Otherwise, 2-3 apps during the growing season should reduce the weeds.