 Why did the alien separate when they go down to the surface instead of working together?
But this vast distance did not mean that isolation could endure forever. Instruments within the ship intercepted radio broadcasts and, within the hour, early TV signals. The history of the planet was tabulated as facts became available. A meteor shower could pass over and their clumsy instruments could interpret it as an all-out enemy attack." "And how long can Earth last? They went much closer to Earth, not intending to commit themselves. For a day they circled the planet, avoiding radar detection, which for them was not difficult, testing, and sampling. Atmospheric analysis showed that as far away as we were." "What's worse is that I now find they also have missiles, range one thousand miles and upward.
 They thought they could cover more ground this way and talk to more people about maintaining peace.
 They want to take advantage of some alone time while they are not on their main ship.  They have different knowledge of these different areas, as they travel to the areas they know more about.  There is not space for two aliens in one small landing craft, so they must split up.  They thought they could cover more ground this way and talk to more people about maintaining peace.