That's not a combination that i would recommend. Roundup kills everything (pretty much). 2 4D is a weed killer that targets specific weeds. It can be used in spraying pastures and such, not killing the crops. That of course is if it is used as directed on the specific crops listed in the instructions with the product. A lot of different yard and pasture products use 2 4D as a main ingredient.
I use Rup & atrazine on field corn. Chems have to be mixed properly to blend correctly or it clumps up and is difficult to clean up. Also, to prevent foaming, water is added from the bottom. Hanging a hose in spray tank foams up bad and contains most of the chem molecules. Read and follow directions on labels.