Their pronunciation and writing are different.

there is nothing you can do that can't be done. cheer...easy go to sourthern china, get a job over there. in hong kong or guangzhou. meeting some new friends who speak these chinese languages.

I've been learning Cantonese for more than five years now and i've only been learning listening and speaking. About a year ago i decided to try learning Mandarin formally through a language institute. I was able to learn it quickly (beginner level Mandarin) and found it just as interesting as Cantonese. However! I quickly found that since i'm not proficient in Cantonese i was moving more into Mandarin and spending less time on Cantonese. They are slighly complementary but only when you know one language (ie Cantonese or Mandarin) really well. If you are a beginner or elementary on both then it will be a battle to learn both at the same time. Pick one Chinese language and then stick with it until you can get a good grasp and then the other one can naturally follow. Just my opinion.

A chinese child can.We cantoness children learn both when young.But I have not seen any foreigner learn both at the same time.

no problem :)