Can people carry on a conversation with each side speaking a different form of Chinese, like in some movies? For example, in the movie Ip Man, the "northerner" martial arts fighter who comes to Foshan to challenge the local martial arts fighters always talks in Mandarin, while everybody he talks to (the local citizens, fighters, Ip Man himself, factory workers, etc.) always speak to him in Cantonese. This is repeated throughout the movie. The conversations are carried on fine with both sides seeming to understand the other perfectly, and both sides assuming that the other side understands them, even though Mandarin and Cantonese are not mutually intelligible. Neither side tries to change to the other side's language, and neither side is annoyed that the other side continues to speak the other language. This seems really odd to me.

Is this realistic?
People can carry on a conversation speaking whatever language they are comfortable with, if they can understand each other's language. This happens ALL the time in immigrant families all over the world. A typical situation is: Parents move from country A to country B, and are native speakers of A but have a good understanding of B. Their children, growing up in country B, acquire B with native fluency and understand A as their home language. In these families, parents and children carry on conversations in two different languages pretty much all the time. I don't see anything unrealistic about it.