Hey,
In my opinion, learn either one first and then another would be better. I have a student who can speak mandarin quite well and then she learnt Cantonese. She said that the class actually helps her in mandarin class too. I don't know if that's true, just her comment. However, I really found that's easier to teach a foreigner who can speak (at least understand) mandarin to speak cantonese or a local who can speak cantonese but to learn mandarin. Both are really a lot quicker than the others.
I don't know if anyone with the experience that once learn a new language and then mix it with the second language sometimes.
I did have sometimes confuse to use mandarin and english in some conversations with different people( some speak mandarin/english only but not both). Another student of mine, who speak fluent spanish (he is native english speaker), he said sometimes when he wants to say something in cantonese but the words pop up in mind that's spanish.
I think that's easy to confuse if you learn a few completely new languages at the same time. Maybe at least with good foundation of a new language and then go for another one.
Just my opinion only. And enjoy learning. That's fun.
