For a Bachelor's you can double-major and often go further with a dual degree (depending on how much of the required coursework you take in the second area).
The question about what to do with the Master's will be completely overshadowed by whether or not you want to do a PhD. You may not need to do one at all or it may be a stepping-stone within your PhD program.
For the PhD, interdisciplinary research is extremely common. You may find programs and certainly advisers that are specifically focused on your interest. If not you can get co-advisors from different departments. This makes your home department more comfortable that someone outside their expertise is there to evaluate and help you. As doctorates are research degrees, not specific skill training like a Master's, you can go pretty far and wide with you thesis topic. But it does need to bring significant novelty as far as your home department is concerned. Just applying methods and ideas from field A to new area B (perhaps no one has done this before) would potentially make a fine thesis in department B, but less so in A.
Note also that multiple doctorates are often not allowed without special permission. Being a "Renaissance person" goes over well as an undergrad, but risks looking unfocused later. Hence it's probably much better to do one interdisciplinary program, versus combine multiple degrees and have to explain how they are complementary, and you aren't just a degree collector or professional student.
Personally, I'd pick the department with the best career prospects to fall back on.