I honestly forget where, but I was once told that if the USA had invaded Japan, the Soviet Union would have as well, resulting in a similar situation as in Berlin. What do you think, would this have happened?
We don't really have a clue what would have happened if the war had gone on. Would the Soviets had tried to invade Hokkaido? Would the US have even had to wage a land invasion of Japan's home islands, or would it have ended before that came to pass? One can invent fictional scenarios in one's head but it is all just fiction. These were not planned out. The Soviet Union did not have complex invasion plans in place; they could have thrown men at Hokkaido (the northernmost Japanese home island), as Stalin did contemplate, but I think the idea that they would somehow neatly divide Japan into two pieces separately by a boundary is rather unlikely to say the least.
It is worth noting, however, that the Berlin Wall was a much later (1960s) solution to a specific "problem" of dual occupation, which is to say the city of Berlin being dual-occupied despite being deep in one state's territory (Eastern Germany). A large literal wall is not how the boundary between East and West Germany generally was enforced, nor the boundary between North and South Korea, nor other "divided" states. So I don't think one needs to literally imagine a large concrete wall dividing Japan either way. There are other ways for states to mark their borders.
People invent all sorts of hypothetical "what would have happened if...?" reasoning for various purposes, often to justify something that happened (e.g., "the atomic bombs were justified, because without them, Japan would be divided by a Berlin Wall"). One should not mistake such things for history. They are an elaborate form of speculative fiction, and have their place in evaluating choices made in the past, but can hardly be taken as being factually grounded.