The presumption is that the major lifeforms on the planet are bipedal, but that there are two different species, with one dominating on each of the continents.
On the volcanic side of the planet, there is a human-like species in a nomadic civilization. This civilization will be based on clans, and their main mode of transportation will be riding a hopping creature. They will travel up the side of the volcano, and before it erupts they will move back down. Each clan or group will have their own special paths up and down the side of the volcano, the way their group has "always" gone. This suggests that, as groups make their way towards the top, they will come into regular conflict with other groups, and that they will have developed an established mechanism for handling conflicts between groups, as well as experience in exploiting the system for their own group's gain.
Prevailing winds will produce a rain shadow in which one side of the volcano will be extremely wet and another side extremely dry. This will also help to ensure these traditional nomadic paths up the volcano eventually conflict, as the best paths will be up the north and south slopes.
The volcano itself will play a large part in their culture. In their creation myths, the volcano spit out the large ball in the sky (the gas giant) which remains always overhead, riding in the sky above the volcano. Animals gathered from closer to the top will have ritual meaning for them. As they climb the mountain they will be passing through niches where different species of food animals will flourish, and the most significant will be the ones gathered from the top of the mountain. Marriages and alliances forged atop the volcano will have special significance.
As they are nomadic, their territorial sense is more portable than static. They don't think of the land as belonging to them, expcept perhaps when they are standing on it.
This species has three sexes: an egg-deliverer, a sperm-deliverer, and a repositor. The repositors will be the rulers; they bring forth life in much the way the volcano brings forth life (by tossing lemmings which haven't left fast enough).
At the top of the volcano are lemming-like creatures who actually make their home in the caldera of the volcano. They have adapted to the volcano, and are sensitive to its eruptions, and pour out of the caldera in the days (weeks?) before it blows. Their emergence does two things: they tear through the surrounding countryside eating the same berries that the humans do, and at the same time they serve as a an "early warning system," telling the humans to get out before the eruption. It's time to migrate elsewhere, quickly. So the clan packs up and moves out.
It's possible that these lemmings have also acquired some sort of floating system, so that when the rocks they are on get expelled by the volcano, they can glide or float long distances. Or perhaps a carapace that would enable them to survive landing after being propelled like buckshot from a volcanic eruption.
On the other side of the planet the climate is a bit colder and more hostile. While this side doesn't have the large volcano, it still does have highlands, caused by the bulge antipodal to the volcano.
The species that develops here is a hopping species, rather than the completely humanoid species from the other side. And the hostile environment drives them further towards technology than their counterparts across the sea, and they are less nomadic and more territorial. They will be more driven to hang on to the things they make, because they will be important to their survival.
They lack the volcano as a visible divine instrument, but because of the activity of the sky above them (due to debris from previous events, including volcanic eruptions, there is a lot of meteor activity over this moon) they believe in an unseen, though perhaps capricious, god (or perhaps a dualism, to account for the apparent randomness on the good/bad coming from sky events).
As this society grows, it will want more land, and it's just a matter of time before the pressure to expand sends them exploring across the ocean and they find the far more friendly environment on the other side of the planet.
The first brave explorers will bring back tales of this wondrous new land, but also bring cautionary tales of this huge light in the middle of the sky, which will make further journeys somewhat daunting, and may have an unsettling effect on them while they are there. Will it fall on them?
Adding to the mix, the species from the non-volcanic side of the planet will be evolutionary cousins to the riding beasts of the nomads. Because of this, they will be susceptible to some of the common diseases of these riding beasts. Since they are related, the riding beasts of the nomads will share some of the basic drives as the intelligent species, and will have a sort of "social cat" psychology. The similarity might lead to a decision by the mounts to rebel and join the invaders.