Go talk to a professor in applied math department. Explore the following - how many additional credits would it take to add applied math as a minor? Or, as a second major? Also inquire about whether you could get admitted into a grad program directly from the CS major, or would you need to take a bunch of prerequisite classes?
Also, are you thinking of taking grad school all the way to a PhD? what do you envision doing with applied math? Heading into academia as a professor or out into the commercial/industrial world?
The answers to each of these should help you make decisions.
If it were me... I would complete the CS degree and a minor in applied math. I would get into the workforce and put these skills to use in the most demanding job I can find. After a couple years of "seasoning" and finding out what you really enjoy and what you think is missing in your education then apply to grad school in an appropriate degree.
By the way, figure out how to manage yourself and build a network for freelance/consulting work and you will be able to easily work your way through grad school by working 15-20 hours a week and not take student loans.