Best path for a PhD in CS from a top school?
Research experience will help you a lot more in getting into a good PhD than an extra degree. For this reason, I would recommend 1. to meet your goals. 

Additionally, having dual degrees in not a requirement for doing inter-disciplinary research. Working with an adviser that has the relevant experiences, taking a few classes, or reading on your own are all great ways to learn about a second field, and then to start to apply your research from your primary field to your secondary field. You can also transition to your secondary field for your PhD if you choose, even if you don't have a degree in that field. My Bachelor's was Math, I am now pursuing a PhD in Computer Science. Doing extra research as an undergraduate helped me more in getting into my program than taking extra CS classes would have.