If you mean that someone has the ability to speak with either of two accents, then I would say to use the accent that will make you more understandable to the people you are with. In the case of my niece, she can speak with either a British or American accent as she has lived back and forth between the two countries all her life. She just adjusts to whatever accent is prevalent in her current location. Another criteria would be for the speaker to use whatever accent is most comfortable provided he or she can be understood by others.

Very few people speak a language with 2 accents. In China, as anywhere else, your accent depends on the province or region you come from. Broadly speaking, most English speakers in China, speak with a British or American accent, depending on who taught them.

Impossible, depending on the regions where they are living, even the native American speak with accent. Spoken American English have several accents, which accent do you want to learn? British? many british watch American movies and episodes, you expect to learn from them for perfect british accent, but you may end up with American accent.

I understand that you find yourself confusing UK and USA styles (in general)? Pick a region (does not have to be a whole country) and focus on that. Eg. North England, or California, or even New Zealand. Any English-speaking region is fine, the important thing is to stick with it. Or specifically, choose a celebrity or person you admire and use that as your starting point. Find music, shows, movies etc from that region. The people you talk to and the media you watch/listen to will influence your own personal accent. Just let it happen. Of course, your teacher and the materials you use to learn would influence how you sound as well. The important thing is to be aware of that, then make your own choices.