If you are able to speak and be intelligible within a decade with both, it will be a minor miracle. Cantonese has more tones than Mandarin as well as an accessory vocabulary. Mandarin is hard enough for any Westerner because they almost never master the tones and sounds. It is mostly through context that most conversations are intelligible. Cantonese is almost unintelligible to me even though understand Mandarin.

ahahah I kinda agree with DP :) In my experience it's never beneficial to learn multiple languages at the same time unless they're so different that you cannot possibly mix them in your brain, and you also happen to have A LOT of time at your disposal. Sharing the same writing system (albeit with some notable differences), mandarin and cantonese could easily create a lot of confusion in your head and slow down your progress in both of them. In my opinion you should focus on the one you presently like the most and save the other for when you'll be advanced enough to avoid any kind of confusion. That being said, do as you please! :) 加油！

It's neither a good or a bad idea. The two languages are so different, anyhow, so you won't confuse them. Right now I'm doing three: Vietnamese, Chinese, and Japanese. I am not confused at all! http://fsi-language-courses.org/Content.php?page=Chinese http://fsi-language-courses.org/Content.php?page=Cantonese You won't get confused with these.

In my experience not a too smart thing to do. Atleast if you start off with both at the same time. I did that, and I kept messing up the vocab and pronunciation... :D

还是学好普通话，走遍天下都不怕。