I grew up learning both English and French at the same time. I think they're a really good combination, if you're going to learn two languages at the same time. There are plenty of cognates, use the same alphabet and are relatively similar with respect to sentence structure. That being said, they're not so similar that you're going to confuse the two when speaking. Learning as a child, however, I found that I was never really fluent in either one. I always say that my first language is "Franglais," as we call it here in Canada.

In my experience, learning two languages at the same time as an adult is completely doable if you're willing to dedicate and immerse yourself. Just keep in mind that it's probably going to take you a much longer time to reach a certain level of fluency in either one.

Best of luck!