People can buy and use both separately. Banning it won't do any good. People who drink too much alcohol+caffeine and get hurt are responsible for that themselves. If they drive and hurt someone else again they chose to take the product. The product didn't do it, and those who refrained from driving didn't harm anyone else.
Individual responsibility is one of the foundations of our society. There would be no way to ban caffeinated alcohol drinks because wherever there is a demand for a product there are people that will find a way to use and sell/make these products. For a good example just look at the history of prohibition.
I believe that a ban on caffeinated alcohol drinks is unnecessary. I believe it is also unwarranted. We already restrict the drinking age to 21, so the proposed ban is focused on adults that are 21 and over. Surely this group is competent to chose their drinks and should be able to purchase caffeinated ones as desired.