Hi Trung, It is true that both of these can mean, 'What will happen next?" (ex: "The little boy eagerly asked his grandpa, 'Then what' when his grandpa came to a pause in his story." ~~ OR ~~ "'What, then, will happen when we run out of fresh water?' asked the student to his teacher."). The phrase "then what" can also be used to ask for alternatives in a comparison (ex: "If I can't use onions in the sauce because you're allergic, then what? [as in what else am I supposed to use]")

They both essentially mean the same thing. "what then?" and "then what?" can both be used to say "What will happen then?" or "What will happen at that time?"