I think as long as you're motivated, you can do both. It really depends what you want to do. I have seen people do both at the same time and I think if you think of them as separate languages, that helps. I would say it helps to have a background in Mandarin if you learn Cantonese only because there's a lot of movies with Chinese subtitles and spoken Cantonese is different from written Cantonese. But I don't think it's absolutely necessary if you're resourceful - you can learn Cantonese from English as well.

of course you can do it. but in China, Cantonese almost used in HK and Guangdong province, not everyone can say it, or need say it, if you just only can speak mandarin you also can walk around China and speak to anyone.

no, it's not bad, it's very convenient