Let's face it: If Mundus wanted to take over just a single planet, he would have done so in an instant. This is a guy who can make countless star systems in a couple seconds with a casual flap of is wings.
Now let's refer to this page:
"The World was born of Darkness"
In this specific statement, there was absolutely nothing but pure darkness. When Mundus creates his place before fighting against Dante, we see that there are countless stars in the sky. Where did those stars come from? Surely there was nothing but pure darkness before the world was created.
With regards to the world separating into two, comparing the human world as a small line compared to the demon world can refer to a single planet, but considering that a line of light happened in the absolute darkness, and the whole thing being split into two "worlds".....
This means that the world of darkness and the world of light are similar in size. Considering that there was
absolutely nothing existing before the human and demon world was created would mean that these "worlds" are not planets, but something much bigger instead. If the world of light meant a solar system or a galaxy, these would have not been born from absolute darkness as there would still
countless numbers of stars/galaxies already existing in the first place.
But nothing existed, at all before the world of light and darkness were created.
By the way, Mundus actually created the space during his fight with Dante. He didn't just teleport himself and Dante there.
Edit: I don't think we refer to "worlds" as solar systems of galaxies here. Only planets or universes usually.